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not fail to be noticed that the remarkable geographical distribution of these
southern lampreys is paralleled by that of certain fresh-water fishes already
described, with the exception that there is no instance among the latter where
a species is common to Australia and South America.

The hag-fishes, of which there are two genera, constitute a

Hag-Fishes. family (Myxinidce) distinguished from the last by the nasal sac

having a posterior duct which perforates the palate ; the single external nasal



554



LOWEST VERTEBRATES.



aperture being situated above the mouth at the extremity of the head, which is
furnished with four pairs of barbels. The mouth is devoid of lips, the palate
is provided with a single median tooth, and there are two comb-like series of
rasping teeth on the tongue. The gill-apertures, or aperture, are situated at a con-
siderable distance from the head ; and each gill-pouch has a separate duct opening
into the oesophagus. The sides of the abdomen carry a row of mucous sacs, and
there is no spiral valve to the intestine. The large eggs are invested in a horny
envelope, furnished with threads for adhesion. In the true hag-fishes, of which
the common species (Myxine glutinosa) is found on the coasts of Europe and
North America, there is but a single gill-opening on each side of the abdomen,
leading by means of six ducts to as many gill-pouches. Another species has been
recorded from the extremity of South America ; and the range of the genus also
includes Japan. In the second genus (Bdellostoma), of which there are two species
from the coasts of the South Pacific, there are six or more gill-openings on each
side, each communicating by a separate duct with a gill-chamber. All these

creatures are marine, and are frequently found deeply
buried in the bodies of fishes, more especially members
of the cod family, into which they bore for the purpose
of feeding on the flesh. They are totally blind, .and
secrete vast quantities of slime, which seriously inter-
feres with fishing in localities where these creatures
abound. Met with in the fjords of Norway at a depth
of about 70 fathoms, hag-fishes have been dredged from
depths of nearly 350 fathoms.

Primeval The Old Red Sandstone of Caithness

Lampreys. h as yielded the skeletons of a small limb-
less creature (Palceospondylus), which there is little
doubt must be regarded as one of the forerunners of
the modern lampreys. Measuring only about a couple
of inches in length, these skeletons show a well-calcified
skull, while the notochord is surrounded by a series of
calcified rings, and the tail has a large fin, of which the
supports on the upper side are forked like those of
lampreys. The front of the head has a circular opening
SLIGHTLY ENLARGED RESTORATION surrounded with a ring of tentacles (dc, lc\ probably
OP THE SKELETON OF THE corresponding to the nose of a modern lamprey; the

PRIMEVAL LAMPREY. After . j.ir./N T_ _J

Traquair. opening ot the nose itself (n) appears to be single ; and

there are a pair of plates (x) behind the head not

improbably representing gill -plates. It may be confidently assumed that this
little creature is but one among a series of lost types.




CHAPTER II.

THE ARMOURED PRIMEVAL VERTEBRATES, Subclass OSTRACOPHORI.

A GROUP of remarkable armoured forms from the Palaeozoic rocks formerly
placed among the fishes, are now regarded as probably constituting a subclass of
the class typically represented by the lampreys, although it must be confessed
that we have at present no evidence of the links which, on this view, must be
supposed to have connected the two groups. Strange in form, and utterly unlike
any living animals, these primeval armoured vertebrates are characterised by the
great development of the external skeleton, the head and fore-part of the body
being protected by large bone-like plates. There are no hard structures to the
mouth, and there are, at most, but rudimental indications of arches for the support
of limbs, while the notochord is persistent. They may be divided into three chief
groups ; and one of the most curious features connected with the first of these is
the apparent structural identity of one layer of its shield with a layer found in
the investment of the living king-crabs.

The simplest of these armoured forms is typified by the genus
Pteraspis of the Devonian rocks, a partial restoration of which is
given in the annexed figure. In these creatures the head and fore-part of the
body are protected both above and beneath by shields; while the tail, in some
cases at least, is scaled. The structure of the shield is curious, each plate consist-
ing of an outer and inner hard layer, between which is a thick stratum of
polygonal chambers, perforated by delicate tunnels of the sensory canal-system
of the skin ; all the
layers lacking the ele-
ments of true bone, and
the outer surface being
marked with fine con-

PARTIAL RESTORATION OF Pteraspis.

Centric striae. Ihe eyes ^ fom A g Woodward, Cat. Foss. Fish. Brit. Mus.)

are lateral and widely

separated, and towards the hinder end of the back shield (which is provided with
a spine) there is an aperture on each side for the escape of water from a gill-
cavity. Although nothing is known as to the form and structure of the nose
and mouth, the nostrils must evidently have been placed near the mouth on the
under surface of the head. A pit between the eyes probably marks the site
of a rudimental median eye ; and the structure of the under surface of the shield
indicates the presence of separated gill-pouches, which were probably supported
by arches. From an examination of the whole structure of the remains, Mr. A. S.
Woodward is of opinion that the shield probably covered " a truly fish-like head ;




556



L O WEST VER TEBRA TES.



Cephalaspis.




and if it was not for the invariable absence of limbs and jaws, and the forward
position of the breathing apparatus in Pteraspis and its allies, these animals might
be placed, without hesitation, in the class of fishes. The possibility that limbs and
jaws were present, but not calcified enough to be preserved, must, however, In-
borne in mind ; while the negative evidence on this subject, and the want of
information as to the nature of the tail, are factors necessitating caution in the
determination of affinities."

The next family of the group is typified by the genus Cephalaspis,
in which the front shield appears to be confined to the head and gill-
region, and consists of a single piece, rounded or pointed in front, abruptly
truncated behind, and with the rounded margin bent inwards below to form an
ornamented flattened rim. Of the triple-layered shield, the inner layer is bony,
the thick middle one solid, although traversed by a network of blood-vessels, while
the upper one is tuberculated and resembles teeth in structure. The eyes are placed
close together in the middle of the shield, the nostrils must have had much the
same position as in Pteraspis, and at the back of the shield there occurs on each
side a small flap which must be regarded as a gill-cover. Immediately behind the

shield commences the
ordinary scaling of
the body, without any
signs of arches for the
support of limbs.
Paired fins appear,
indeed, to be totally
absent, although a dorsal and a caudal fin, stiffened by little elongated scales in
place of rays, are present. The large, deep, quadrangular scales covering the body
form a series of interlocking rings, doubtless corresponding in the living state to
the underlying muscle-plates of the body.

The third modification of the group, as represented by the
Devonian Pterichthys, agrees in the general structure of the shield
with certain members of the last section in which there is no dividing line between
the head-shield and the united scales of the body. The head is, however, sharply
defined from the body ; and the armour, instead of being simple, consists of a
number of overlapping plates arranged symmetrically to one another. An
important point of distinction from
all the preceding forms is to be
found in the presence of a pair of
hollow limb-like pectoral append-
ages, jointed near the middle. A
small movable plate between the
eyes seems to have lodged a median
eye; another movable plate on the RESTORATION OF PTERICHTHYS. ( From Traqn air.)

cheek appears to represent the gill-
cover ; and a pair of loose jaw-plates on the lower surface of the front of the head,
in some forms at least, are finely toothed on the hinder border; but nothing
definite is known with regard to the nature of the nose, mouth, and jaws.



RESTORATION OP Cephalaspis. (From Woodward. Cat. Foss. Fish. Brit. Mus.)



Pterlchthys.




EXTINCT ARMOURED FORMS.



557



The arrangement of the median fins is generally similar to that obtaining in
the second family. With regard to the true nature of the pectoral appendages.
Mr. Woodward writes that they are commonly considered " as homologous with
the paired pectoral fins of fishes, and certainly in their mode of fixation to the
trunk there is much to favour that supposition; but the discovery of an allied
organism in the Devonian of Spitzbergen suggests the wisdom of suspending
judgment. The dorsal body -shield of the latter is simpler than that of
Pterichtkys, but the arrangement of the plates on the ventral aspect is identical,
and there are also hollow pectoral appendages. These appendages, however, are
merely simple triangular spines, firmly fixed by suture to processes of the body-
armour ; and if they are an inferior or special condition of the ordinarily movable
arms, it seems not unlikely that those arms will prove to be different in origin
from the paired limbs of fishes."



CHAPTER III.

THE LANCELETS, Subkingdom PBOTOCHORDATA.
Class Leptocardii.





COMMON LANCELET, SINGLY, AND IN A CHAIN ( nat. size).

WITH the curious semitransparent little creatures known as lancelets, forming
the only family (Branchiostomatidce) and genus of the class to which they belong,
we leave the Vertebrates and come to the lower group of Protochordates ; all of
which retain the three essential vertebrate features mentioned on p. 549. First
described by the German naturalist Pallas in 1778, from a specimen captured on
the Cornish coast, the common lancelet (Branchiostoma lanceolatum) was referred
to that refuge for the destitute, the Mollusca, where it remained till 1834, when it
was rediscovered by Costa, on the Neapolitan coast, who gave the name of
Branchiostoma, and placed it among the fishes, in the neighbourhood of the
lampreys and hags. It was again discovered by Yarrell in 1836, who assigned the
title of Amphioxus, and was the first to recognise the existence of a cartilaginous
vertebral column, or notochord. The upper figure of our illustration shows the
pointed extremities of the body, and also a number of chevron-shaped lines, with
their angles directed forwards, these being the partitions dividing the longitudinal
mass of muscle clothing each side of the body into a series of segments. And it is
due to this segmented structure that the lancelet is enabled to swim so speedily as
it does, its progress being effected by serpentine movements of the body. Paired
fins are wanting ; but the back is provided with a continuous dorsal fin, expanded
posteriorly into a caudal fin, and continued forwards to join the ring of feelers, or



LANCELETS. 559

tentacles, growing from the margin of the hood-like expansion of skin which
surrounds the mouth. The notochord extends to the anterior and posterior
extremities of the body, reaching beyond the muscle-plates, and likewise in advance
of the front extremity of the overlying nerve-chord; the latter feature being
peculiar to the lancelet. An aperture distant about two-thirds of the whole length
from the head, and opening in the middle line of the lower surface of the body, is
the outlet of a large cavity, or atrial chamber, surrounding most of the internal
organs, and especially the large pharynx ; and the vent, as in many tadpoles, is
situated high up on the left side, near the hinder end of the body. The reproduc-
tive organs, which form oval structures lying below the muscle-plates, differ from
those of the Vertebrates in that they consist of a large number of perfectly distinct
chambers, corresponding to the muscle-segments of the region of the body along
which they extend. In connection with the fins, it should be observed that, except
at its two extremities, the dorsal fin is supported by a series of gelatinous rays, each
lying in a chamber of its own ; while the ventral portion of the caudal fin has a paired
series of similar supports. In young and transparent examples, the pharynx, or that
portion of the alimentary tract immediately behind the mouth, is distinctly visible
through the walls of the body, and can be seen to be perforated on each side by a very
large number of vertical gill-slits, opening into the atrial chamber. In the living
creature an almost continuous current of water is drawn, for the purpose of breath-
ing and feeding, through the mouth into the pharynx, whence it escapes by means of
the gill-slits into the atrial chamber, from which it is discharged through the pore.
Unlike even the lowest Vertebrates, lancelets have no cartilaginous skull ; the only
solid structure in the head taking the form of a ring of cartilage in the hood surround-
ing the mouth, which gives off a series of processes for the support of the feelers.
Although paired eyes, as well as organs of hearing, are totally wanting in these
strange little creatures, a pigment-spot at the front end of the nerve-tube represents
a median eye ; behind which is a small nasal pit, communicating in the larva by
means of a small pore with the front of the nerve-tube. With regard to the other
soft- parts, it will suffice to mention that the anterior extremity of the nerve-tube
is not expanded to form a true brain ; and that the heart is represented merely by
a series of pulsating dilatations of the great blood-vessel ; the blood itself being
devoid of colour.

Lancelets are represented by some eight or nine species, all of which may be
included in a single genus ; although one from the Bahamas is peculiar on account
of the unsymmetrical arrangement of its reproductive organs. Essentially littoral
forms, inhabiting shallow water, especially where the bottom is sandy, these
creatures have an almost universal distribution on the temperate and tropical
coasts, although they are often curiously local. The European form has been
recorded from Scandinavia, Heligoland, the English Channel, France, the Medi-
terranean, and Chesapeake Bay, growing to an unusual size in French waters.
Other species occur on the Atlantic and Pacific shores of North and South
America, as well as on the coasts of Australia, Japan, Ceylon, and the Fiji Islands.
Mr. A. Willey remarks that the lancelet " possesses an extraordinary capacity for
burrowing in the sand of the seashore or sea-bottom. If an individual be dropped
from the hand on to a mound of wet sand, which has just been dredged out of the



5 6o SEMIVERTEBRATES.

water, it will burrow its way to the lowest depths of the sand-hillock in the
twinkling of an eye. Its usual modus vivendi is to bury the whole of its body in
the sand, leaving only the mouth with the expanded buccal cirri [tentacles] pro-
truding. When obtained in this position in a glass jar, a constant inflowing current
of water, in which food-particles are involved, can be observed in the neighbour-
hood of the upstanding mouths. The food consists almost entirely of microscopic
plants (diatoms, desmids, etc.) and vegetable debris . . . Occasionally it emerges
from its favourite position in the sand, and after swimming about for some time it
will sink to the bottom, and there recline for a longer or shorter period upon its
side on the surface of the sand. When resting on the sand, it is unable to maintain
its equilibrium in the same position as an ordinary fish would do, but invariably
topples over on its side, indifferently, the right or left ; " this inability to maintain
its balance being due to the absence of certain structures of the internal ear, to
which this function is assigned in fishes. According to another observer, lancelets
occasionally attach themselves to another by their mouths in a chain-like manner,
as represented in our illustration. That lancelets indicate an extremely archaic
type, and also that they are more nearly allied to the Vertebrates than to the
Invertebrates, may be considered certain ; although there is still a difference of
opinion whether they should be looked upon as simple or degraded forms.



CHAPTER IV.

THE SEA-SQUIRTS OR ASCIDIANS, Class Tunicata.




A LEATHERY FIXED SEA-SQUIRT, M 1C rOCOSmuS (liat. size).

EXTERNALLY, scarcely any creatures are more unlike the lancelet than those
fixed marine animals commonly known as sea-squirts, and technically as asciclians,
or tunicates. Nevertheless, in the opinion of those best qualified to judge, the
relationship is probably closer than that existing between the former animal and
the larva of a lamprey, in spite of the much greater external resemblance between
the two latter. It is, however, when we dissect a sea-squirt that we meet with
structures recalling certain features in the anatomy of the lancelet ; while to find
evidence of the chordate affinities of the former, we have to go back to its larval
condition. In the adult condition, writes Mr. Willey, most of the sea-squirts " are
sedentary animals, remaining fixed for their lifetime on one spot, whether attached
to rocks, stones, shells, or seaweeds, from which they are incapable of moving.
There are, however, several very extraordinary genera of ascidians, which swim or
float about perpetually in the open ocean, and have become adapted in the extremest
VOL. v. 36



562 SEMIVERTEBRATES.

manner to a purely pelagic environment." As there are both simple and compound
fixed ascidians, so there are two similar types among the pelagic forms; but some
of the latter are complicated by an alternation of generations, the one generation
being a simple form, whereas in the other generation the units are aggregated into
Chains as shown in our Plate of the creatures known as salpse. Among the
compound fixed types the colonies, as they are termed, consist of a number of
individuals produced by budding from a single parent-stock ; such colonies frequently
attaining very large dimensions, and being remarkable for their brilliant coloration,
although in other cases they merely form thin incrustations on the surface of various
marine objects. Other forms, 'on the contrary, are merely connected at their bases
by a common creeping root-like base, from which new buds are from time to time
given off, the individuals being otherwise free. 1




A LEATHERY SEA-SQUIRT, WITH ONE SIDE OF THE OUTER TUNIC REMOVED (liat. size).

structure of Externally a simple sea-squirt, like the one (A. microcosmus) re-

Ascidians. presented in the first illustration, has been aptly compared to a leather
bottle with two spouts; these spouts forming funnel-shaped projections, one of
which generally situated at a higher level than the other takes in water, which is
discharged from the second. The whole organism is invested in an external tunic,
varying much in structure, but being frequently warty, and generally opaque,
although in the salpaB it is transparent. A remarkable feature connected with
this outer tunic is that it contains a substance cellulose identical in composition
with that forming the cell-walls of plant-tissues. On cutting through the outer
tunic, we come, as in our second illustration, to an underlying muscular tunic,
forming the true body-wall, and consisting externally of an epidermis underlain
by interlacing muscular fibres. In the illustration, a indicates the iiihalent, and
b the exhalent orifice of this inner tunic. On cutting into the inner tunic, we find
a large so-called atrial cavity, enclosing to a great extent the viscera, and com-
municating with the exterior by means of the exhalent orifice. The inhalent
orifice, or mouth, communicates, on the other hand, directly with the exceedingly

1 Strictly speaking, the term "individual" includes all the units produced by budding from a common stock,
but it is more convenient to use it in the ordinary sense.



SEA-SQUIRTS. 563

large pharynx or branchial chamber, which extends nearly to the hinder end of
the body, and is perforated by a vast number of gill-openings, through which the
water taken in at the mouth passes into the atrial chamber. Instead of passing
directly into the latter chamber with the water, the food is caught up in a mass of
slime, and carried round the base of the mouth-tube until it ivacln>s the nitrance
to the oesophagus, which lies near the hinder end of the dorsal surface of the
branchial chamber. Hence it passes into the stomach, and along the intestine,
which forms a U-shaped curve turned away from the dorsal aspect; the vent
opening on the same aspect into the atrial cavity below the exhalent orifice. With
regard to the nervous and circulatory system, it will suffice to say that there is a
large nerve-ganglion embedded in the tissue of the inner tunic, and lying on the
dorsal surface of the body between the inhalent and exhalent orifices ; and true
blood-vessels are wanting, the blood merely flowing through a series of spaces in
the muscles and other tissues of the body and between the viscera, and the heart
forming a dilated tube. Unlike the higher Chordates, all the ascidians are
hermaphrodite ; the reproductive organs frequently lying within the loop of the
intestine, and discharging into the atrial cavity alongside of the vent. A remark-
able physiological feature of the group is to be found in the periodical reversal
of the action of the heart; the blood being driven for a certain time in one
direction, after which the heart makes a short pause, and then propels it in an
opposite course.

In addition to certain other structural features, into the consideration of which
it would be impossible to enter in a work of the present nature, the essential
resemblance between the adult sea-squirts and the lancelets is to be found in the
possession by both of a pharynx perforated by a large number of gill-openings,
which convert it into a branchial chamber, opening into an atrial cavity instead of
directly to the exterior. Several of the differences between the two, such as the
hermaphrodite reproduction and the bent intestine of the sea-squirts, are probably
due to their sessile habits, since such features are characteristic of most fixed
organisms. Other points of difference are to be found in the absence of segmentation,
and the want of a dorsal nerve-tube and notochord in the adult ascidian, although,
as we shall see, a remnant of the latter exists in the tail during the larval condition.
All ascidians, whether fixed or free in the adult condition, go
through a free-swimming larval stage, during a part of which they
develop a tail containing a notochord and nerve-tube ; and as this feature is all
important from a morphological point of view, it must be mentioned here, although
necessarily in a very brief manner. Generally the larval condition lasts but a
short time; and this may be the reason for the development of the tail, as a
powerful swimming organ would seem to be essential in order to enable the
creature to reach a spot suitable for its permanent existence. During its develop-
ment a groove makes its appearance on one surface of the ascidian embryo, the
large cells on the side of which grow inwards so as to enclose a tube, corresponding
to the nerve-tube of Vertebrates, beneath which is the notochord. When of an
oval shape, and while still contained in its investing membrane, the embryo assumes
a ventral curvature, and at the same time produces a long tapering tail, which
eventually becomes coiled round it. In addition to certain other structures, this



5 6 4



SEMI VERTEBRA TES.



outgrowing tail includes the nerve-tube and the notochord ; and in some forms
contains the only muscles developed at all. Subsequently a rudimentary brain,
corresponding to a simple structure in the lancelet, makes its appearance; and
likewise an unpaired eye, agreeing precisely in structure and mode of develop-
ment with the rudimental median eye of the tuatera. After certain other
changes, among which the development of a stomach and intestine are included,
the. larva is ready to burst from its membranes, which it does by spasmodic




Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 59 of 62)