IX 



MOLLUSCA 



353 



hoUowB out to form the kidney, whilst a band of cells grows out 

 from each and forms a ring round the gut, splits into two layers, 

 and forms the pericardium. 



The accounts of Harms and Lillie leave no doubt in tlie mind that 

 in the embryo of the Unionidae we are merely dealing with an 

 ordinary Pelecypod veliger modified for a parasitic existence. 



Fig. 284. — Transverse section of a Glochidium larva of Unio which is already fixed in 

 the tissues of its po.st. (After Harm. ) 



a,m, cells which will form the adult mantle ; int, intestine: ^.m, vacuolated cells of larval mantle, 

 which .absorb food material from the host ; m,c, mantle-cavity ; p.£r, rudiments of pedal ganglit ; t, teetli 

 of larval shell. 



CEPHALOPODA — Loligo, Sepia 



We must now turn to the study of the embryology of the highest 

 MoUusca, the Cephalopoda (lit. head-footed), so called because the 

 fore part of the foot has grown into a frill surrounding the head. 

 Two genera are represented by common species, both on the English 

 coasts and in the Mediterranean ; these are Sepia officinalis and 

 Loligo vulgaris. So far as is known, the development of both pursues 

 a practically identical course. We shall select- Loligo as "a type for 

 special study, because its development has been more completely 

 worked out, and because species of this genus are common on the 

 American coast ; but we shall not hesitate to fill up lacunae in our 

 . knowledge of the development of Loligo by the description of corre- 

 sponding stages from the development of Sepia, when these are 

 better known. 



The eggs of both genera, like all Cephalopod eggs so far described, 

 contain an abundance of yolk, the cytoplasm being mainly restricted 

 to a small disc at the animal pole of the egg, in which the nucleus is 



VOL. I 2 a 



