Page:EB1911 - Volume 18.djvu/704

 development, being only in a condition common to it and other Coelomata. It now passes to the veliger phase, a definitely molluscan form, in which the disproportion between the area in front of the ciliated circlet and that behind it is very greatly increased, so that the former is now simply an emarginated region of the head fringed with cilia. It is termed the “velum,” and is frequently drawn out into lobes and processes. As in the Rotifera, it serves the veliger larva as an organ of locomotion. The body of the veliger is characterized by the development of the visceral hump on one surface, and by that of the foot on the other. Growth is greater in the vertical dorso-ventral axis than in the longitudinal oro-anal axis; consequently the foot is relatively small and projects as a blunt process between mouth and anus, which are not widely distant from one another, whilst the antipedal area projects in the form of a great hump or dome. In the centre of this antipedal area there has appeared (often at a very early period) a gland-like depression or follicle of the integument. This is the primitive shell-sac discovered by Lankester in 1871, and shown by him to precede the development of the permanent shell in a variety of molluscan types. The shell-gland is bounded by a ridge of ectodermic cells. This ridge forms the edge of the shell-secreting epithelium, and therefore of the mantle, since the shell extends to the edge of the mantle. The shell-gland, as development proceeds, extends from its point of origin as an ectodermic thickening, which may be only slightly concave or may be deeply invaginated and then evaginated.

In the larvae of several Gastropoda and Lamellibranchia occur excretory organs which have the characters of true nephridia. There is a single pair of these organs situated immediately behind the velum. They agree with primitive nephridia in being of ectodermic origin, in consisting of perforated cells in linear series, and in having no communication with the coelom. The inner end of each of these organs consists of a flame-cell, i.e. a cell with an internal cavity containing a vibrating filament or flagellum. They are best developed in the Pulmonata; in some cases they are very rudimentary and may be destitute of an external opening. They invariably disappear before the adult stage is reached, but their presence in the larva is evidence that the ancestral mollusc possessed a pair of true nephridia quite distinct from the coelomic excretory organs, which are so characteristic of existing forms in the adult condition.

The ctenidia, it will be observed, have not yet been mentioned, and they are indeed the last of the characteristic Molluscan organs to make their appearance. They arise as outgrowths of the sides of the body within the cavity formed by the development of the mantle. The veliger, as soon as its shell has attained some extent and begins to assume definite shape, is no longer of a form common

to Mollusca generally, but acquires characters peculiar to the particular class to which its parents belong. For the later development therefore the articles on the several classes must be consulted.

Relations between the Classes.—From the preceding discussion an idea may be formed of the primitive characters of the Phylum Mollusca, and it is possible to construct a diagrammatic mollusc, as was first done by Lankester, which will possess these primitive features. The figure here given represents such a hypothetical form according to present views. We cannot assert that this was in all respects the condition of the common ancestor, as will be seen when we attempt to derive the various sub-types from it. In the Amphineura the nervous system, having no separate ganglia and no ventral visceral commissure, may be still more primitive. The metameric repetition of the shell-plates and of the ctenidia are probably special modifications, but it is difficult to explain the spicules of the dorsal integument except as a condition more primitive than the shell itself. The Prorhipidoglossomorpha are distinguished by the separation of the genital coelom from the pericardium, and by the long visceral commissure passing ventral to the intestine. The Lamellibranchia have markedly diverged from the original type by the adoption of filtration as a method of feeding. This has