DEVELOPMENT OF THE EXTERNAL FORM — PROSOBRANCHIA. 149 



formation of the germ-layers but also the development of the external 

 shape of the body. This is the ease, for instance, in Nassa, Fusus, 

 Fulgur, Natim and others. Even in Veiinetus, the Veliger stage of 

 which we became acquainted with (Fig. ")">), the Trochophore form is 

 no longer distinctly developed. The velum appears at first in the form 

 of two wavy cell-bands at the anterior end of the ventral surface and 

 near it appear the rudiments of the tentacles ; immediately behind 

 them are the mouth and the pedal swelling. The latter appears as 

 a rudiment when the velum is only slightly developed and is far 

 from complete dorsally. The rudiments of the organs, with the 

 exception of the dorsally placed shell-gland, are thus here crowded 

 together into a very limited area of the very large embryo. This is 

 the case to a far greater extent when the egg is still richer in yolk, 

 as, for instance, in Fulgur (McMurrich, No. 70). The first rudi- 

 ments of the organs are here so crowded together that we might 

 almost speak of a germ-disc in contrast to the large yolk-mass of 

 the egg. We should then see the commencement of processes which, 

 in a far higher degree, will be met with in the Cephalopoda. Thus 

 in eggs very rich in yolk we may speak of a " blastoderm " which 

 grows round the yolk, i.e., the macromeres, and, indeed, the layer of 

 micromeres is here greatly reduced as compared with the yolk-mass 

 of the macromeres, as may be seen by a glance at Fig. 42 D and E, 

 p. 112, and Fig. 47 A and B, p. 117. If we compare these figures with 

 those of the blastula and invagination-gastrula of Patella (Figs. 49 

 and 50), Planorbis or PalwUna, it is evident that these altered condi- 

 tions must bring with them modifications in the external shape of 

 the body. 



In Nassa mutabili*, which we select for description as the best 

 investigated if not the most extreme form in this respect, there is 

 a point at the vegetative pole which remains for some time uncovered 

 l>\ cells (Figs. 61 ^4, hi, and 47 C, bp). This is the blastopore which 

 closes later, the stomodaeum arising in this region (Fig. 47 D, »>). 

 In Fusus, the eggs of which exhibit a similar condition, the blastopore 

 is said to persist and to pass over into the mouth (Bobretzky). 

 The toot appears very early as a broad swelling behind the blastopore, 

 even before the rudiment of the velum has arisen (Fig. 61 A, f). 

 Near it lie the groups of ectoderm-cells {ex) which have been claimed 

 as an excretory apparatus (external kidney). The velum (r) appears 

 in front of the blastopore, advancing from the ventral to the dorsal 

 Bide. Dorsally, the shell-gland appears, and over it the shell-integu- 

 ment. At a later stage, the anterior part together with the foot 



