without. The PEDATA include the HolothuridsB with peltate tentacles.
FIG. 338. Cucumaria frondosa, sea cucumber. (From Emerton.)
Holothuria * in warmer waters, one species furnishing the trepang of
Chinese markets. The CUCUMARIID^E represented in our waters by Cucu-
maria * (Pentacta) with regular rows of ambulacra, Thyone * with them
scattered, and Psolus* scaly with a creeping disc. The deep-sea ELA-
SIPODA belong to the Pedata. The APODA are represented by Gaudina *
(fig. 336) and Molpadia*
Order II. Paractinopoda.
No radial canals nor ambulacra. Tentacular canals arising from ring
canal. Myriotrochus* Synapla* Oligotrochus* (fig. 339).
Summary of Important Facts.
1. The ECHINODERMA share the radiate structure with the
Coelenterata, but differ from them (a) in the numerical basis of
the symmetry (five) ; (b) in that, as embryology shows, they have
descended from bilateral forms.
2. Farther characters are the existence of a coelom, the
ambulacral system, and the mesodermal spiny skeleton, which has
given the name to the phylum.
3. The ambulacral S3^stem is locomotor and occurs nowhere else.
It consists of a sieve-like plate, the madreporite (not always pres-
ent), which passes water to the stone canal, and from this to the
FIG 339,Oligotrochus vitreus.* (After Danielssen and Koren.)
ring canal and the radial canals to fill the ampullae and ambulacra.
Lateral branches supply the tentacles and cause their extension.
4. Blood-vessels and nerve cords run in the same radii as the
radial canals of the ambulacral system; stone canal, madreporite,
ovoid gland, and genital ducts are interradial.
5. The Echinoderma are divided into five classes: (1) Aster-
oidea, (2) Ophiuroidea, (3) Crinoidea, (4) Echinoidea, and (5)
6. The ASTEROIDEA have a disc and (usually) five arms into
which the gastric pouches and hepatic caeca extend. The ambu-
lacral groove open.
7. The OPHIUROIDEA also have disc and arms, but the ambu-
lacral groove is closed and the hepatic caeca absent.
8. The CRINOIDEA have a cup-shaped body bearing arms,
usually branching, with pinnulae, and a stalk, usually with cirri.
They are either temporarily or permanently attached. The
Crinoidea are subdivided into Eucrinoidea, Edrioasteroidea,
Cystidea, and Blastoidea.
9. The ECHINOIDEA are usually spherical or oval, armored with
calcareous plates which extend as meridional bands from peristome
to periproct, five pairs of ambulacral and five of interambulacral.
10. The ambulacral plates end at the periproct with a single
ocular plate; the interambulacral with a similar genital plate.
The madreporite is fused with one of the genital plates.
MOLLUSC A. 351
11. The regular sea urchins have the anus in the centre of the
periproct, the mouth in the peristome; the ambulacral areas are
12. The Clypeastroidea have a central mouth, the anus outside
the periproct in the posterior interradius; the ambulacral areas
13. The Spatangoidea are markedly bilateral, the mouth an-
terior, the anus posterior; ambulacral areas petaloid.
14. The HOLOTHUROIDEA are elongate and worm-like; the
skeletal system greatly reduced; they are more or less bilaterally
symmetrical and have usually a single gonad and two branchial
trees. They are divided into Actinopoda, with radial canals, and
PHYLUM VI. MOLLUSCA.
At the first glance the molluscs, like the flatworms and leeches,
give the impression of parenchymatous animals. A spacious coelom
is absent; what was formerly regarded as a body cavity is a system
of sinuses surrounding the viscera and connected with the blood
system, and is especially developed in the Acephala. More recently
the view has gained ground that the molluscs have descended
from ccelomate animals, and from forms in which, by encroach-
ments of a connective tissue and muscular parenchyma, the coelom
has been reduced to the inconspicuous remnants of the pericardium
and the lumen of the gonads.
Where the molluscan organization is well developed, as in the
snails, four parts may be recognized in the body (fig. 340), The
visceral sac forms the chief mass of the body; it is less rich in
muscles than the rest because it is reduced to a thin peripheral
layer by the alimentary canal, liver, nephridia, and gonads. In
front it is continuous with the head, which, according to the group,
is more or less marked off by a neck, and bears, besides the mouth,
the tentacles and eyes, the most important sense organs. Below,
the visceral sac passes into a muscular mass, usually used for loco-
motion, the foot. From the back extends the pallium or mantle,
a dermal fold which envelops a goodly part of the body. The
Acephala (fig. 340, C) have a double mantle, right and left, both
halves springing from the dorsal line and extending down over the
visceral sac and foot. The cephalopods (fig. 340, A) and the snails
(fig. 340, B), on the other hand, have an unpaired mantle which
arises from about the central part of the back and either extends
MOLL U8C A.
down on all sides or, like a cowl, covers either the anterior or
posterior parts of the body. The mantle is of importance in two
ways : its outer surface is covered with epithelium which, like that
of the adjacent surface, has the power of secreting shell, a thick
cuticular layer of organic matter (conchiolin) largely impregnated
with calcic carbonate. The inner surface of the mantle, together
FIG. 340. Diagrams of three molluscan classes. A. a cephalopod (Sepia) ; B, a gas-
teropod (Helix); C, an acephal (Anodontd). a, anus; c, cerebral ganglion; /it, foot;
m, mantle chamber; .sc/i, shell; , siphon; v, visceral ganglion. Visceral sac
dotted; mantle lined, shell black.
with the outer surface of the body, bounds a space, the mantle cavity,
which, from its most important function, is also called the bran-
chial chamber. Since most molluscs are aquatic, special vascular
processes of the body, the gills or branchiae, lie in this space; in
the terrestrial forms its walls serve as lungs and thus are respiratory.
From the foregoing it will be seen that the character of the
mantle must exert an influence on the shape of the shell and on
the respiratory organs. Paired mantle folds necessitate two valves,
right and left, to the shell; a right and left branchial chamber,
and right and left gills. With an unpaired mantle the shell is
always unpaired, while the gills may retain their primitive paired
The gills in the mantle cavity are called ctenidia, from their resem-
blance to combs with two rows of teeth. Each consists of an axial portion
(back of the comb), containing the chief blood-vessels and two rows of
branchial leaves. The whole is united to the wall of the branchial cavity
by the axis (fig. 385). In many aquatic forms the ctenidia are lacking,
and then the respiration is either diffuse by the skin or by accessory gills
which by structure (usually outside the mantle cavity) are distinguished
from the ctenidia.
Those parts of the surface of the mollusc which are not covered
by the shell have a columnar epithelium which is frequently ciliated
and which contains unicellular mucus glands, especially abundant
on the edge of the mantle. These give these animals the soft slip-
pery skin which is implied in the name Mollusca (mollis, soft).
Many-celled glands, like the byssus gland of the Acephala, the
pedal gland of many snails, occur.
Although the existence of head, foot, and mantle is very char-
acteristic of the molluscs, they are not always present. In the
Acephala there is no distinct head region; many gasteropods lack
the mantle and hence the shell; in the Cephalopoda the foot is
converted into other appendages, the siphon and arms. These
modifications are to be explained by degeneration and evolution.
In the nervous system are also some highly characteristic features.
As a rule it consists of three pairs of ganglia associated with
important sense organs and connected by nerve cords. One pair
lies dorsal to the oesophagus and corresponds to the supraoesophageal
ganglion of the worms; it is the brain (cerebrum) and supplies
Fia. 341. Nervous systems of Molluscs. A, most gasteropods; J9, acephals; C, cepha-
lopods and pulmonates. c, cerebral; pa, parietal, pe, pedal, pi, pleural, and v,
the tentacles and eyes. A second pair lies ventral to the alimentary
tract on the front part of the muscle mass of the foot : these are
the pedal ganglia which are connected with the otocysts. The
third pair, the visceral ganglia, are also ventral, and near them
are the third sense organs, which are widely distributeed through
the Mollusca, and which from position and structure are regarded
354 MOLLUSC A.
as organs of smell (osphradia). They are thickened patches of
ciliated epithelia extending into the mantle cavity. Pedal and
visceral ganglia are united to the cerebrum by nerve cords, the
cerebropedal and cerebrovisceral connectives respectively. Accord-
ingly as these connectives are long or short the ganglia are wide
apart or united into a nerve mass around the oesophagus.
Primitive Mollusca (Amphineura) have a simpler condition. The
cerebral ganglia lie dorsal to the oesophagus and are united by a cord
around the oesophagus (fig. 344). From it are given off two pairs of lat-
eral nerve tracts, the ventral or pedal cords, and lateral or pleural cords,
the latter united by a loop dorsal to the anus. By a concentration of
ganglion cells the pedal cords give rise to the pedal ganglia, and similarly
the pleural cords form three pairs of ganglia, the pleural and the parietal,
as well as the visceral already mentioned, of the cerebrovisceral cord (fig.
341, A). The pleural ganglia are connected with the pedal by nerve cords;
the parietal innervates the osphradium. When farther concentration takes
place the pleural may unite with the cerebral, and the parietal with the
visceral (fig. 341, B), or both may fuse with the visceral (C). In the latter
case the visceral ganglion (in the wider sense) is associated with the pedal
by the pleuropedal connective ; while in the other the connective is appa-
rently absent because fused with the cerebropedal. Although the otocyst
receives its nerve from the pedal ganglion, the centre of innervation lies in
The heart, which lies dorsally, consists of auricles and ven-
tricles. The ventricle is always unpaired, but there are two auricles
where two gills exist from which the blood flows to the heart, but
with the loss of one gill one auricle may disappear. Distinct arteries
and veins occur; capillaries are found only in the Cephalopoda,
while in the lower molluscs, and especially in the Acephala, the
smaller arteries open into lacunar spaces which were formerly
regarded as the body cavity. A completely closed vascular system
does not exist even in the Cephalopoda.
The heart is enclosed in a spacious sac or pericardium, which,
with few exceptions, is connected with the nephridia by a ciliated
canal, and in many molluscs (Cephalopoda and some Acephala) is
also related to the gonads. These facts support the view, already
mentioned, that the pericardium and the lumen of the gonads are
the remnants of the coelom; for here, as in the annelids, the
nephridia open by ciliated nephrostomes into the ccelom, and the
sexual cells arise either from the coelomic walls or from sacs cut
off from them. Even more important for this view would be
confirmation of the disputed statement that in Paludina vivipara
the coelom (enteroccele) arises as diverticula from the archenteron.
MOLLUSC A. 355
Nephridia and sexual organs are primitively paired, but fre-
quently are single by the degeneration of the structures of one side.
The animals are either hermaphroditic or dioecious, but the gonads
are always very large. Even more room in the visceral sac is
demanded by the digestive tract in which oesophagus, stomach, a
coiled intestine, a voluminous liver, and frequently salivary glands
may be recognized. The radula or lingual ribbon is also a char-
acteristic organ, and its absence from the Acephala is probably to-
be explained by degeneration. It is a plate or band armed with
teeth which lies on the floor of the pharynx on a ventral ridge,,
the tongue, and is used for the communication of food (figs. 366,.
Keproduction is exclusively sexual; budding, fission, or parthen-
ogenesis have not yet been observed. The eggs, united in large
numbers, are usually enveloped in jelly and are either rich in
deutoplasm or are enveloped in a nourishing albumen. A few~
molluscs (e.g., Paludina vivipara) are viviparous. A metamor-
FIG. 342. Veliger larva (trochophore) of Teredo navalte. (From Hatschek.) A, anus:
J, stomach ; J^ intestine; , liver; LM.d, LM.v, dorsal and ventral longitudinal
muscles ; Meg, primitive mesoderm cells ; JfP, teloblast ; Neph, protonephros ; O,
mouth ; Oe, oesophagus ; R, rectum ; S, shell ; ScM, hinge ; SM.h, SM.v, posterior
and anterior adductors; S, apical plate; Wkr, wkr. pre- and postoral ciliated
bands ; uvs, cilia of apical plate.
phosis is of wide occurrence. In such cases a 'veliger' larva
escapes from the egg (fig. 342) ; in this can be recognized head,
foot, and mantle, even in those cases where one or the other of
these is lacking in the adult. This shows that the absence of
mantle, shell, or head, which occur in large groups of molluscs,
is not a primitive condition, but can only be explained by degen-
eration. The name veliger arises from the velum, a strong circle
of cilia, which surrounds a frontal or velar field in front of the
mouth, and which serves as a locomotor organ for the larva. In
some cases (fig. 343 B} it is lobed like the trochus of a Rotifer.
Fia. 343. Veliger stages. A, of a snail; B, of a Pteropod. (From Gegenbaur.)
o, shell; op, operculum ; p, foot ; t, tentacle; v, velum.
The veliger recalls the annelid trochophore and serves for the
distribution of the species; it is therefore of great importance for
animals which, like most molluscs, are sedentary or slow-moving.
In cases without metamorphosis (Cephalopoda, Pulmonata, etc.)
the veliger stage is frequently indicated during embryonic devel-
opment by a ridge of cells surrounding a preoral velar field.
Class I. Amphineura.
These forms, some of which appear in the Silurian, are clearly
the most primitive of molluscs, and are distinguished by a marked
bilateral symmetry. The nervous system already described (p.
354) consists of pleural and pedal cords with scattered ganglion
cells and no ganglia, these cords being connected by numerous
commissures (fig. 344, B}.
Sub Class I. PlacopJiora (CMtonidce).
The chitons were formerly included among the gasteropods
because of the presence of a creeping foot and a radula. They
are at a glance distinguished from them by the rudimentary con-
dition of the head and the shell. This last is unique among mol-
luscs; it consists of eight transverse plates overlapping like shingles,
which allows the animal to roll itself into a ball. The edge of the
mantle extends beyond the shell and is covered with spines, while
in the mantle cavity beneath are, right and left, a series of ctenidia.
Nerves enter the shell and end with noticeable sense organs (aes-
FiG.344. Chiton squamosus, dorsal view. (After Haller.) A, the entire animal; /?,
after removal of shell and viscera, a, anus; C', brain; K, ctenidia; o, mouth;
P, pedal nerve cord; pi, pleurovisceral nerve cord.
thetes and, in some, eyes, fig. 345). The symmetry of the body is
also expressed in the viscera. The anus is medial, and right and
FIG. 345. Eye and aesthetes of Acanthopleura spiniger. (After Moseley.) a, macrges-
thete; b, micrassthete; /, calcareous cornea; gr, lens; /i, iris; fc, pigmented cap-
sule ; n, p, nerves ; ?, retina.
left of it are the openings of the nephridia and sexual organs. The
sexes are separate, the gonads unpaired, while corresponding to the
paired arrangement of the gills there are two auricles to the heart.
The Chitons are represented on our northeastern coast by several
small species (Trachydermon,* Auricula*}; farther south and on the
Pacific shores are larger species (Cryptochiton *).
358 MOLLUSC A.
Sub Class II. Solenogastres (Aplacophora).
Worm-like forms without shell; the
foot rudimentary and at the bottom of
a ventral groove. The radula is also
reduced; in Chcetoderma it bears but a
single tooth. The gills are either small
or wanting. The usually hermaphrodite
animals have the gonads emptying into
an unpaired chamber (pericardium?) and
FIG. m.-Neomenia corinata, thence to the exterior by the paired
BSS^IS^^SS nephridia. Clmtoderma in New Eng-
terior;c, ventral groove. land; ^eomenia, Dondersia.
Class II. Acephala (Lamellibranchiata, Pelecypoda).
These have, among the molluscs, the least powers of locomo-
tion. Some are fixed, the majority burrow slowly through sand
or mud ; only a few spring by means of the foot or swim by open-
ing or closing the shells. Hence it is that they need more pro-
tection than other species, and this is afforded by the strong shells
in which the body can usually be completely enclosed. This shell
recalls that of the brachiopod in that it consists of two halves or
valves, but these valves are right and left rather than dorsal and
ventral, and hence are usually symmetrical in shape. Only when
the animal rests permanently on the right or left side is this sym-
metry lost, and then the symmetry of the soft parts is affected.
The two lobes of the mantle which secrete the shell on their
outer surface arise from the back of the animal and grow down-
wards, forwards, and backwards, so that they envelop the whole
-(fig. 352). Hence the oldest and the most thickened part of the
.shell, the umbo, occurs near the back (fig, 347). Around this
the lines of growth are arranged concentrically, lines which show
how, by gradual growth of the mantle, the shell has increased in
.size. On the back the valves approach each other, and in the
majority are movably connected by a hinge, which consists of
projections (' teeth ') in one valve fitting into depressions in the
other. In the Brachiopoda the valves are opened by appropriate
muscles; in the Acephala by an elastic hinge ligament usually placed
dorsal to and behind the hinge. The shell is closed by adductor
muscles which extend through the body from shell to shell, leav-
ing their impressions or scars on the inner surface (fig. 347).
Usually there occur an anterior and a posterior adductor equally
well developed (Dimyaria); less frequently the anterior is rudi-
mentary (Heteromyaria) or entirely disappears (Monomyaria).
FIG. 347. Left valve of Crassatella plumhea, inner and outer surfaces. (From Zittel.)
The outer surface showing lines of growth ; no pallial sinus.
FIG. 348. Right valve of Mactra gtuitorwm, with pallial sinus. (From Lud wig-Leunis.)
Letters for both figures: a', anterior; a", posterior adductor scar; e, hinge; I,
internal ligamental groove ; m, pallial line ; s, pallial sinus.
When the muscles are relaxed (as always occurs at death) the elastic
ligament opens the valves.
The heterodont hinge is the typical form (fig. 348); each valve bears a
group of teeth near the umbo, those of the left alternating with those of
the right. Besides these ' cardinal teeth ' there are in front and behind
'lateral teeth? often produced into ridges. The ligament lies behind the
hinge and is usually visible from the outside (external ligament), but is
occasionally transferred to the interior (internal ligament, fig. 347). The so-
called schizodont and desmodont hinges are modifications of the hetero-
dont. Then there are Acephala of apparently primitive character which
either lack the hinge (dysodont), or have one composed of numerous teeth
in a series symmetrical to the umbo (taxodont), or of two strong teeth like-
wise symmetrical to the umbo (isodont). In these cases the ligament is
developed in front of as well as behind the umbo, and may be either
external or internal.
Since the secretion of shell takes place most rapidly at the edge
of the mantle, both are closely united, the union being strength-
ened by small muscles. So the edge of the shell has a different
appearance from the rest, this part being marked off by a pallial
line parallel to the margin (fig. 347). In many species, the Sinu-
palliata, the line at the hinder end makes a large bay (pallial
sinus) (fig. 348, s). Since the mantle folds are membranes with
free margins, it follows that when the shell is closed these edges are
pressed together, which would prevent the free entrance and exit
of water. To accommodate this each mantle has its margin exca-
vated at the posterior end, so that when brought together two
openings, an upper and a lower, result (fig. 349, 0). The lower
FlG. 349. Ventral views of siphonate and asiphonate acephals. A, Anodonta cygnea ,'
B, Isocardia cor ; (7, Lutraria elliptica. a, anal siphon ; />, branchial siphon ;/, foot;
A;', outer, A.", inner gill lamella; wi, mantle; s, shell.
of these is the branchial opening by which fresh water passes into
the mantle (branchial) chamber; it flows out after passing over
the gills, along with the faeces, through the upper or cloacal open-
ing. In many bivalves the free edges of the mantle grow together,
FIG. 350. Section of shell of Aru
;. c, cuticula; p, prismatic layer; I, nacreous
leaving three openings, one for the protrusion of the foot, the
others the two just described, which are now called the incurrent
(branchial) and excurrent (cloacal) siphons (fig. 349, B). By
further development the margins of these openings are drawn out
into two long conjoined tubes (fig. 349, A}, which for their retrac-
tion need special muscles, which are attached to the valves and
thus cause the pallial sinus referred to above (fig. 348).
In the shell three layers may be distinguished (fig. 350) : on the outside
a thin organic cuticula and below two layers largely of calcic carbonate.
In many these two layers are distinguished as the prismatic layer and the
nacreous layer, the first consisting of closely packed prisms; the nacreous
layer of thin lamellae generally parallel to the surface. These by their free
edges produce diffraction spectra and so the iridescent appearance of the
shell; the finer the lines thus formed the more beautiful the play of colors.
This is especially noticeable in the mother-of-pearl shells Meleagrina and
Margaritina margaritifera. When foreign substances get between mantle
and shell they stimulate a greater secretion of nacreous substance and
become surrounded by layers of it. In this way pearls are formed.
K? K 3
FIG. 351. Anatomy of Anodonta, the mantle, gill, and liver of the right side
removed, the pericardium opened. 1, 2, anterior and posterior adductors; I, II,
III, cerebral, pedal, and visceral ganglia; a, anus; 7. 1 , ib 2 , upper and lower limbs
of organs of Bojanus ; frr, branchial siphon; d, intestine; e, nephridial opening;
fu, foot ; 0, gonad ; Ti 1 , ft 2 , ventricle and auricle of heart ; 7c', insertion of both
lamellae of right gill ; fc 3 , 7c 4 , inner and outer lamellae of left gill; Z, left liver ; I',
its opening in ?n, stomach; rn7, pallial line; r 1 , anterior, r a , posterior retractor
muscle: sp, nephrostome : v, labial palpus. The arrows show the planes of sec-
tion of fig. 352.
The gills lie between the mantle and the body and from their
lamellar character have given rise to the name Lamellibranchiata.
(figs. 351, 352). Two gill-leaves occur on either side, although
occasionally the outer or both may degenerate. Frequently the
gills of the two sides unite behind the body and produce a parti-
tion which separates the mantle cavity into a small dorsal cloacal
chamber and the larger lower respiratory cavity. Into the cloaca
empty the anus and the water which has passed over the gills; it
opens to the exterior through the excurrent siphon. The incurrent
siphon leads into the branchial chamber. In front of the gills
are two more pairs of leaf -like lobes, the labial palpi, between
which is the mouth.
The gills are variously developed. The Nuculidse the most primitive
of living Acephala have true ctenidia consisting of an axis grown to
the body and an inner and an outer row of gill leaves (fig. 355). From