PHYLOGENY OF THE PELECYPODA. 



289 



fessor Ilatsclic'lc's fii;-iuTs (3G-i")3) of dovflopiiii*- Eu|)oina1us (Sorpula) shows a close 

 similarity in structure, pi-oving the identity of the two tyi)es at this early stage. 



The name typembryo Professor Hyatt a])plies to the period in developing animals when 

 the characteristics of the greater divisions begin to appear; when the embryo has char- 

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

 of molluscs to be the veliger stage, at Avhich 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 typembryo 

 ]3eriod in the mollusca. The shell-gland wliich ]iroduces the first rudiments of the prim- 

 itive shell is considered characteristic of all molluscs {vide p. 297). 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 mollusca; 

 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 mollusca 

 as claimed by Professor Hyatt, but also to that class of mollusca to which the embi-yo 

 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 spirally 

 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 woidd therefore suggest an alter- 

 ation in Professor Hyatt's definition of the typembi-yo. In the mollusca the typembryo 

 is the period characterized hy the existence of a shell-gland and the plate-like heginnings 

 of a shell; in it normally the ciliated ti"ochosphere 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 phjdnm to 

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

 gest for it the name 2)hyleml)ryo} The phylrmhryonic mollusc is an early period, in de- 

 velopment, yet having characteis in the shell and anatomy which make 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 jii-otoconch and prodissoconch of molluscs is a period later than that 

 at which the phylenibryonic features are fixed and yet is very distinct from the period 



Lankcster's and Balfour's figures of the trochosphere stage 

 liave already acquired a shell gland and in regard to tliis 

 character cannot be considered as purely trochospheres ac- 

 cording to Lankester's definition, for, with the shell gland 

 they acquire a feature which makes them referable to the 

 mollusca. They are trochospheres in which characters 

 typically occurring later are very early Inherited so as to 

 cause an overlapping of stages. 



I ip-^Xir/, a race, tribe stock, phylum, and "E/JiJ/ivav, 

 an embryo. 



' The existence of a velum cannot be used very closely 

 as a diagnostic character in considering stages of gro\vth. 

 It is frequently nearly or quite absent, as in Unio, Anodon, 

 PI. XXIII, figs. 7-8, Cyclas, and Spharium, PI. xsx, fig. 

 i, or it may exist long after the period of its normal 

 disappearance. In studies of developing St/c<4(ii»is canal- 

 iculattis, PI. xxm, fig. 20, I have found that a large, 

 highly developed velum exists for a considerable time af- 

 ter the formation of the true shell, although it is lost be- 

 fore the young leave the egg cases. 



