238 GASTEROPODA. 



Pleurobranchidium (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 

 form the shell-gland, and shortly afterwards the velum and foot are formed 

 in the normal way, and a stomodaeum appears close to the ventral edge of 

 the velum (fig. 101). The two yolk cells (ry) still remain distinct, but a 

 true hypoblastic layer (probably derived from them, though this has not 

 been made out) soon becomes established. Prominent cells early make 

 their appearance at the base of the foot, which become at a later period 

 invaginated to form the anus. Otolithic sacks (of) become formed in the 

 foot, and the supraoesophageal ganglia from a differentiation of the epiblast 



(ng\ 



At a later period the shell-gland becomes everted, and a nautiloid shell 

 developed. The alimentary tract becomes completed, though the two yolk 

 cells long retain their original distinctness. The shell-muscle is developed, 

 and peculiar pigmented bodies are formed below the velum. The foot 

 becomes prominent and acquires an operculum. 



The metamorphosis of Tergipes has been more or less completely worked 

 out by Nordmann and by Schultze (No. 271). 



In Tergipes Edwardsii worked out by the former author, the larva when 

 hatched is provided with a large velum, eyes, tentacles, an elongated 

 operculated foot, and mantle. In the next stage both shell and operculum 

 are thrown off, and the body becomes elongated and pointed behind. Still 

 later a pair of gill-processes with hepatic diverticula becomes formed. 



The velum next becomes reduced, and two small processes, which give 

 rise to the lip tentacles and a second pair of gills, sprout out. An ecdysis 

 now takes place, and leads to further changes which soon result in the 

 attainment of the adult form. 



In Tergipes lacinulatus, observed by Schultze, the velum atrophies before 

 the shell and operculum are thrown off. 



Pulmonata. The development of the fresh-water Pulmonata appears 

 from Lankester's observations on the pond-snail (Lymnaeus) to be very 

 similar in all important particulars to that of marine Branchiogasteropoda. 

 The velum is however less developed than in most marine forms. The 

 shell-gland, etc. have the normal development. In Lymnaeus the blasto- 

 pore has an elongated form and it is still a matter of dispute whether it 

 closes at the mouth or anus. 



In the Helicidae there is a gastrula by epibole. The shell-gland, as 

 may be gathered from Von Jhering's figures, has the usual form, and an 

 external shell of the usual larval type is developed. There is a ciliated 

 process above the mouth, which extends into the lumen of the mouth. This 

 process is often regarded as a rudimentary velum, but probably has not this 

 value. There is no other organ which can be homologous with the velum. 



