in 



MOLLUSC A. 



archetype, and thai to do away with a great part of iU utility in 

 exhibiting the tendencies of every Mollusc. All that appean to be 

 really necessary is to show : fint, that no molluacan form present* 

 feature* in it* development which cannot be reconciled with the 

 archetype ; and secondly, that the kind of modification! which hare 

 been supposed to take place in the conversion of the archetype into 

 the special type* are such u actually occur. 



The fint stage of development of the Molluica resembles that of 

 other animsls, The yelk, at first a homogeneous mass, undergoes the 

 process of division to a greater or less extent, it* outermost layers 

 eventually becoming converted into a blastodermic layer, the plastic 

 material out of which the future animal U modelled. 



Flit. 12. 



Development of I. Olatrlina. 11. Lamttltiranch (Lori-n). III. Anliopa. 

 IV. Srpia (KulUkcrJ. 



a, oral ipcrtore ; b, anal aperture, or ritrcmitj of the Intestine ; d, cloacal 

 aperture and atrium (Aftcldians) ; tp, rpipodlum ; ml, mctapudiutn ; g t hypo- 

 l>harrni*al band (Aacldiana) ; m, mantle ; ', ", anal and branchial siphon* 

 (LomrlMrancHata) ; I, branch!* ; A, anterior adductor (Lamrllibranchiata) ; 

 //, posterior adductor. 



In the Molltucoiila the rounded or oval embryo thus formed either 

 become* covered with cilia and swims away as a free form (Polyzoa, 

 Jlratkiopodat), or it gives rise from one portion of its surface to a long 

 fin-like muscular process (fig. 12, 1. 1.), by whose rapid vibration it U pro- 

 jelled (A*cidioiila, in great part). With what organ of the MoUuica is 

 this ' tail ' or ' fin ' of the Ascidian larva homologous ? This is a very 

 difficult point to ascertain, as the tail arises before the regions of the 

 animal are differentiated. At first sight one might be tempted to 

 consider it as a modification of the velum of the embryos of the 

 MuUiaca proper ; but it* relation to the middle of the neural surface, 

 and its insertion close behind the ganglion, which may be readily 

 observed in later stages, appear rather to indicate that it is the homo- 

 logue of the foot proper, and probably of the metapodium, as this is 

 the portion of the foot which in the Mollutca appears fint 



In the further development of the MolltucMa there can bo no 

 question that, as regards the Polytoa, the neural region soon almost 

 oaves to grow, the further increase of the body taking place by the 

 disproportionate development of the htemal region, which constitutes 

 almost the whole of the body of the adult animal, and presents the 

 surface by which it becomes fixed. Again, simple inspection U sum 



cient to show that the intestine extends into the great abdomen thus 

 developed ; that it acquires herewith a neural flexure ; that the ten- 

 tacles are produced from the margins of its oral aperture ; and that 

 the pharynx acquires a Urge proportionate size. 



In the Atcidioida the neural region remains in a like rudimentary 

 condition, the hieiual region undergoing a similar disproportionate 

 growth ; but it is next to impossible to ascertain from the study of 

 development whether this biomal outgrowth is formed behind the 

 anus or before it, inasmuch as the intestine has acquired its complete 

 bicmal flexure when its parU are fint distinguishable. 



In the youngest state in which the different organs are distinguish- 

 able, the intestine is almost entirely bent up on to the lucuial side of 

 the body; the pharynx is a wide cavity (not wider proportionally 

 however than that of a Polyzoon); the tentacles spring from its 

 margin in exactly the same relative position as in a Polyzoon, and 

 there is no atrial cavity. By degrees the pharyngeal cavity enlarges still 

 more, the tentacles remaining comparatively rudimentary (jig. 12, i. 2). 

 Contemporaneously with these changes, the end of the intestine become* 

 more and more bent down towards the neural surface, and a cavity, 

 which in another Mollusc would be the mantle-cavity, appears around 

 its extremity ; a single or two lateral apertures (subsequently uniting 

 into one) are soon formed, and allow this cloacal portion of the atrial 

 cavitjuto communicate Kith the exterior. At the same time the atrium 

 extends on each side of the enlarged pharynx, detaching it from the 

 side of the body, and enveloping it just as a serous sac invests the 

 surface of a viscus. Ciliated apertures (at first one or two only on 

 each side) now pierce the wall of the enlarged pharynx, and increase 

 in number until it assumes the structure of the perfect branchial sac. 

 Finally, it depends upon the proportional development of the branchial 

 sac, and of the post-abdomen, whether the adult Ascidian shall belong 

 to the Branchial or to the Intestinal subtype. 



We unfortunately know hardly anything of the development of the 

 Brackiopoda ; but so for as, the Polyzoa and Atcidioida are concerned, 

 it is obvious that the hypothetical modifications of the Archetype do 

 in fact faithfully represent the actual course of development (See 

 however the remarks, further on, as to the nature of the post-abdominal 

 outgrowth in haemal AloUutcoida and Mulluica.) 



Development of the Neural MoUutca. The Lamellibranchiata. The 

 first step towards the production of the organs from the blastodermic 

 layer in this group is the development of one portion of its surface into 

 a disc with mixed edges provided with very long cilia (Jig. 1 2, u. 1 ). Next 

 in the inner substance of the germ the intestine appean as a solid man, 

 bent upon itself, towards what the eventual development of the foot 

 proves to be the neural surface ; its oral portion being placed imme- 

 diately behind the ciliated disc (2). Finally, the haomol surface behind 

 the ciliated disc gradually gives rise to the two lobes of the mantle, 

 upon each of which a thin transparent pellicle, the first rudiment of 

 one valve of the shell, eventually appears. As development goes on 

 (3), the neural surface between the primarily approximated oral and 

 anal apertures becomes converted into the large foot andfaneaoaoma of 

 the LameUibranchs, which serve to lodge the principal mass of the 

 viscera, the abdomen never becoming developed into a great process 

 as in Gasteropoda. The great posterior adductor makes iU appear- 

 ance on the neural side of the intestine, and by its development the 

 latter is thrown up so as almost to appear to have a hivmal flexure. 

 The gills next appear as processes of the body within the mantle- 

 cavity, and therefore have not the remotest homology with the 

 pharyngeal branchial sac of Ascidions, any more than the two 

 siphoual apertures which are essentially dependent upon the union of 

 the two lobes of the mantle with the gills and with one another have 

 anything to do with the oral and cloacal apertures of the Ascidiana. 



Finally, it is said that the ciliated disc becomes metamorphosed into 

 the labial palpi. This is a point well worthy of further investigation ; for 

 the arrangement and form of the appendages in Pecten lead us strongly 

 to believe, as we have said, that they are the homologues of the tenta- 

 cles in the Atculioida and J'oly:oa. On the other hand, there can be 

 no doubt that the ciliated disc of Lamellibranchs is homologous with 

 the ciliated lobes of the Uasteropod embryos; and these, there is 

 every reason to believe, are nothiug but the specially modified anterior 

 portion of the epipodium. The tentacles of the Polyzoa would thus 

 come to be the homologues of the epipodium ; but the validity of the 

 whole chain of reasoning obviously dependa upou whether the ciliated 

 disc does or doe* not become metamorphosed into the palpi a position 

 which the more requires confirmation as in the Gasteropoda the ciliated 

 lobes are now known entirely to disappear. However this may be, 

 what has been stated with regard to the main steps in the develop- 

 ment of the LameUibranchiata fully confirms the hypothetical deri- 

 vation of the type from the Common Plan. 



Ptaopoda and Puimonata. In the primary stages of their develop- 

 ment no important distinction U to be drawn between the members 

 of this division and those of the last, except that in the Pteropoda the 

 ciliated disc is replaced by two ciliated lobes, one on each side ; and 

 in the I'ulmonate embryos by a contractile expansion their so- 

 called 'yelk-sac.' The primarily neural flexure of the intestine in 

 the Pulmonala, and the development of their mantle in front of the 

 anus (that is, the development of an abdomen), are fully demonstrated 

 by late observations upou their embryogeny. It is important to 

 remark, that in the Pteropoda the ciliated lobes of the embryo do not 



