Page:EB1911 - Volume 11.djvu/532

Fig. 26.—Development of the River-Snail, Paludina vivipara. (After Lankester, 17.)
dc,	Directive corpuscle (outcast cell).
ae,	Arch-enteron or cavity lined by the enteric cell-layer or endoderm.
bl,	Blastopore.
vr,	Velum or circlet of ciliated cells.
dv,	Velar area or cephalic dome.
sm,	Site of the as yet unformed mouth.
f,	Foot.
mes,	Rudiments of the skeleto-trophic tissues.
pi,	The pedicle of invagination, the future rectum.
shgl,	The primitive shell-sac or shell-gland.
m,	Mouth.
an,	Anus.
A, Diblastula phase (optical section). B, The diblastula has become a trochosphere by the development of the ciliated ring vr (optical section). C, Side view of the trochosphere with commencing formation of the foot. D, Further advanced trochosphere (optical section). E, The trochosphere passing to the veliger stage, dorsal view showing the formation of the primitive shell-sac. F, Side view of the same, showing foot, shell-sac (shgl), velum (vr), mouth and anus. N.B.—In this development the blastopore is not elongated; it persists as the anus. The mouth and stomodaeum form independently of the blastopore.

One further feature of the development of the Pectinibranchia deserves special mention. Many Gastropoda deposit their eggs, after fertilization, enclosed in capsules; others, as Paludina, are viviparous; others, again, as the Zygobranchia, agree with the Lamellibranch Conchifera (the bivalves) in having simple exits for the ova without glandular walls, and therefore discharge their eggs unenclosed in capsules freely into the sea-water; such unencapsuled eggs are merely enclosed each in its own delicate chorion. When egg-capsules are formed they are often of large size, have tough walls, and in each capsule are several eggs floating in a viscid fluid. In some cases all the eggs in a capsule develop; in other cases one egg only in a capsule (Neritina), or a small proportion (Purpura, Buccinum), advance in development; the rest are arrested either after the first process of cell-division (cleavage) or before that process. The arrested embryos or eggs are then swallowed and digested by those in the same capsule which have advanced in development. This is clearly the same process in essence as that of the formation of a vitellogenous gland from part of the primitive ovary, or of the feeding of an ovarian egg by the absorption of neighbouring potential eggs; but here the period at which the sacrifice of one egg to another takes place is somewhat late. What it is that determines the arrest of some eggs and the progressive development of others in the same capsule is at present unknown.

Fig. 27.—Oxygyrus Keraudrenii.
(From Owen.)
a,	Mouth and odontophore.
b,	Cephalic tentacles.
c,	Eye.
d,	Propodium (B) and mesopodium.
e,	Metapodium.
f,	Operculum.
h,	Mantle-chamber.
i,	Ctenidium (gill-plume).
k,	Retractor muscle of foot.
l,	Optic tentacle.
m,	Stomach.
n,	Dorsal surface overhung by the mantle-skirt; the letter is close to the salivary gland.
o,	Rectum and anus.
p,	Liver.
q,	Renal organ (nephridium).
s,	Ventricle.
u,	The otocyst attached to the cerebral ganglion.
w,	Testis.
x,	Auricle of the heart.
y,	Vesicle on genital duct.
z,	Penis.

In the tribe of Pectinibranchia called Heteropoda the foot takes the form of a swimming organ. The nervous system and sense organs are highly developed. The odontophore also is remarkably developed, its lateral teeth being mobile, and it serves as an efficient organ for attacking the other pelagic forms on which the Heteropoda prey. The sexes are distinct, as in all Streptoneura; and genital ducts and accessory glands and pouches are present, as in all Pectinibranchia. The Heteropoda exhibit a series of modifications in the form and proportions of the visceral mass and foot, leading from a condition readily comparable with that of a typical Pectinibranch such as Rostellaria, with the three regions of the foot strongly marked and a coiled visceral hump of the usual proportions, up to a condition in which the whole body is of a tapering cylindrical shape, the foot a plate-like vertical fin, and the visceral hump almost completely atrophied. Three steps of this modification may be distinguished as three families:—Atlantidae, Carinariidae and Pterotrachaeidae. They are true Pectinibranchia which have taken to a pelagic life, and the peculiarities of structure which they exhibit are strictly adaptations consequent upon their changed mode of life. Such adaptations are the transparency and colourlessness of the tissues, and the modifications of the foot, which still shows in Atlanta the form common in Pectinibranchia (compare fig. 27 and fig. 24). The cylindrical body of Pterotrachaea is paralleled by the slug-like forms of Euthyneura. J. W. Spengel has shown that the visceral loop of the Heteropoda is streptoneurous. Special to the Heteropoda is the high elaboration of the lingual ribbon, and, as an agreement with some of the opisthobranchiate Euthyneura, but as a difference from the Pectinibranchia, we find the otocysts closely attached to the cerebral ganglia. This is, however, less of a difference than it was at one time supposed to be, for it has been shown by H. Lacaze-Duthiers, and also by F. Leydig, that the otocysts of Pectinibranchia even when lying close upon the pedal ganglion (as in fig. 21) yet receive their special nerve (which can sometimes be readily isolated) from the cerebral ganglion (see fig. 11). Accordingly the difference is one of position of the otocyst and not of its nerve-supply. The Heteropoda are further remarkable for the high development of their cephalic eyes, and for the typical character of their osphradium (Spengel’s olfactory organ). This is a groove, the edges of which are raised and ciliated, lying near the branchial plume in the genera which possess that organ, whilst in Firoloida, which has no branchial plume, the osphradium occupies a corresponding position. Beneath the ciliated groove is placed an elongated ganglion (olfactory ganglion) connected by a nerve to the supra-intestinal (therefore the primitively dextral) ganglion of the long