196 GASTEROPODA. 



By a change in the relations of the various parts and especially by 

 the growth of the posterior region of the body, the velum now occupies a 

 position at the end of the body opposite the blastopore. Immediately 

 behind it there appear two organs, one on the dorsal and one on the 

 ventral side. That 011 the dorsal side (sh) is a deep pit the shell-gland 

 which is continuous with a layer of columnar epiblast which ends near the 

 anus. The other organ (s), situated 011 the ventral side, is a simple de- 

 pression, and is the rudiment of the stomodseum. Between it and the 

 dorsally placed anus is a slight prominence the rudiment of the foot. On 

 the two sides of the body, between the epiblast and hypoblast on a level 

 with the shell gland are placed two masses of excretory cells, the pro- 

 visional kidneys (D, x). These are probably not homologous with the 

 provisional renal organ of Nassa and other marine Prosobranchiata. At 

 a later period a ciliated cavity appears in them, which probably com- 

 municates with the exterior at the side of the throat. 



In the later stages the foot grows rapidly, and forms a very prominent 

 mass between the mouth and the anus. An operculurn is developed some- 

 what late in a shallow groove lined by thickened epiblast. 



A provisional chitinous plug is formed in the shell-gland which soon 

 becomes everted. The shell is formed in the usual way on the everted 

 surface of the shell-gland. The thickened edge of this part becomes the 

 edge of the mantle, and soon projects in the neighbourhood of the anus as 

 a marked fold. 



With the rapid growth of the larva the invaginated mesenteron becomes 

 relatively reduced in size. In its central part yolk spherules become 

 deposited, while the part adjoining the blastopore (anus) becomes elongated 

 to give rise to the intestine. The stomodseum grows greatly in length and 

 joins the dorsal part of the archenteron which then becomes the stomach. 

 The part of the mesenteron with yolk spherules forms the liver. With 

 the development of the visceral sack the anus shifts its position. It first 

 passes somewhat to the left, and is then carried completely to the right. 



The development of Entoconcha mirabilis (Joh. Miiller, No. 265), a 

 remarkable Prosobranchiate parasitic in the body cavity of Synapta, which 

 in the adult state is reduced to little more than an hermaphrodite generative 

 sack, deserves a short description. It is viviparous, and the ovum gives 

 rise to a larva which from the hardly sufficient chai-acters of the foot and 

 shell is supposed to be related to Natica. 



There is nothing very striking in the development. The food yolk is 

 scanty. The velum, as might be anticipated from the viviparous develop- 

 ment, is small. The tentacles are placed not within, but behind the velar 

 area. There is a natica-like shell, a large mantle- cavity, and a large two- 

 lobed foot. 



In Purpura, Buccinum, and Neritina only one out of the many ova 

 included in each egg-capsule develops. The rest atrophy and are used as 

 food by the one which develops. 



Opisthobranchiata. It will be convenient to take a species of Pleuro- 

 branchidium (Aplysia), observed by Lankester (No. 239), as a type of 

 Nudibranchiate development. The ovum first divides into two segments, 

 and from these small segments are budded off, which gradually grow 

 round and enclose the two large segments. The small segments now form 

 the epiblast. 



At the aboral pole the epiblast becomes thickened and invaginated to 



