94 ART. 4. — N. YATSU : 



in a dorso-lateral position, along the dorsal furrow formed by the 

 reflected part of the arm-apparatus and the oesophagus. 



Differences between the larvae of the 15 p, c. stage and the 

 young LiiHfula (4.5-9 mm. in shell length). — In order to complete 

 my present studies I made sections of several young specimens of 

 IAngula (4.5-9 mm. in shell length) ; one of the median sagittal 

 sections is diagrammatically represented in the accompanying cut. 

 A glance at the cut shows on one hand that the young differs but 

 little from the adult, and on the other that it has undergone 

 not a little change since the oldest stage (15 p.c. stage) I was 

 able to rear from the free swimming larvae. I shall, therefore, 

 briefly enumerate the chief changes that have taken place during 

 the intervening stages : — In the young IAngula (4.5-9 mm.). 



1). The shell has come to be composed of two or three 

 layers of alternate cuticular and calcareous parts. 



2). The palliai sinuses have greatly developed in the mantle. 

 The seUe have increased both in number and in size. 



3). In the lateral body walls the parietal muscle layers 

 have greatly thickened. Both on the ventral and dorsal body 

 walls we can see the specialized portions from which the spindle 

 bodies are formed. 



4). The blood corpuscle and spindle bodies have increased 

 in number. The leucocytes have made their appearance. 



5). The mid-gut and the intestine have elongated exceed- 

 ingly ; the latter turns dorsally and after making a loop ter- 

 minates in the anus. The liver lobes have been constricted and 

 drawn out into the hepatic ducts, the stomach being formed. 



6). The gastroparietal band has increased in breadth, and 

 strong muscle fibres have been laid. The ileo-parietal band has 



