PHYLOGENY OF THE PELECYPODA. ' 289 



fessor Hatscliek's figures (36-53) of developing' Eupoinatiis (Sei'pula) shows a close 

 similarity in structure, proving the identity of the two types at this early stage. 



The name typembryo Professor Hyatt ap])lies to the period in developing animals when 

 the characteristics of the greater divisions begin to appear; when the embr3'o has char- 

 acteristic features of the sub-kingdom to which it belongs. He considers the typembiyo 

 of molluscs to be the veliger stage, at which period the embryo is sufficiently far ad- 

 vanced to be recognizable as a mollusc. He speaks of the protoconch (completed embry- 

 onic shell) as a typembryo. 



I think Professor Hyatt has not given the necessary limitations to his typembi-yo 

 period in the moUusca. The shell-gland which produces the first rudiments of the prim- 

 itive shell is considered characteristic of all molluscs {vide p. 207). It is the first defi- 

 nitely molluscan feature to appear, and as it is singularly alike in all molluscs where 

 known, the embryo at the period when the shell-gland exists is referable to the mollnsca; 

 but has no characters yet which refer it to the class or phylum to which it properly be- 

 longs. The veliger stage, however, is referable, not only to the sub-kingdom mollnsca 

 as claimed by Professor Hyatt, but also to that class of mollnsca to which the embryo 

 really belongs. The veligei- Pelecypod, fig. 25, p. 300, PI. xxiii, figs. 9-11, has two valves 

 and an anterior adductor muscle; the veliger Gasteropod, PI. xxiii, fig. 19, has a si)irally 

 twisted shell and characteristic foot, tentacles and eyes; the veliger Scaphopod, PI. xxiii, 

 figs. 13-14, has a saddle-shaped shell known nowhere else in developing molluscs; the 

 veliger Pteropod has an anatomy peculiar to itself. I would therefore suggest an alter- 

 ation in Pi'ofessor Hyatt's definition of the typembryo. In the mollnsca the tiipemhriio 

 is the jyeriod characterized by the existence of a shell-gland and the plate-like heginnings 

 of a shell; in it normally the ciliated trochosphere disc is becoming transformed into the 

 velum and the foot is developing. Examples of typembryo molluscs are given in the 

 summary of stages later. 



I have shown that the veliger stage of molluscs is referable to the class or jihyhnn to 

 which the embryo really Ijelongs. As it is a period most distinct and important, I sug- 

 gest for it the name phylembryo} The phylenibryonic mollusc is an early period in de- 

 velopment, yet having characters in the shell and anatom.y which mxike it referable to the 

 class to which it belongs. Commonly a velum exists at this stage; but the stage is not 

 meant to include the whole veliger period.'^ Examples of phylembryos are given in the 

 summary of stages. 



The completed ])i-otoconch and prodissoconch of molluscs is a jjeriod later than that 

 at which the phylembryonic features ai'e fixed and yet is very distinct from the period 



Lankester's and Balfour's figures of the trocliosphere stage ^ Tiie existeuce of a velum cannot be used very closely 

 have already acquired a shell gland and in regard to tliis as a diagnostic cliaracter In considering stages of growtli. 

 character cannot be considered as purely trochosplieres ac- M is fre([uently nearly or quite absent, as iu Unio, Anodon, 

 cording to Lankester's definition, for, -with the shell gland I'l. xxiii, figs. 7-8, Cyclas, and Sphrerium, PI. xxx, fig. 

 they acquire a feature which makes them referable to the 4, or it may exist long after the period of its normal 

 moUusca. They are trochospheres in which characters disappearance. In studies of developing Si/cotopiis caiial- 

 typically occurring later are very early inherited so as to ictilatus, PI. xxrn, fig. 20, I have found that a large, 

 cause an overlapping of stages. highly developed velum exists for a considerable time af- 

 i yvAiiy, a race, tribe stock, pliyUuii, and "A7/,?//vi<v, ter the formation of the true shell, although it is lost be- 

 an embryo. fore the young leave the egg cases. 



