62 



part of tlio blastopore iuwards, and its ultimate destiny 

 is to form the ojDeuing from the (ectodeimic) stomodoeum 

 into the (endodermic) mesenteron. The remainder of the 

 blastopore has closed. On either side of the blastopore 

 in its slit-like stage we find one of the mesoderm cells, 

 which divides and gives rise to a pair of rounded 

 elevations. These two elevations are the rudiments of 

 the foot. 



At this stage the larva is practically a trochosphere. 

 Its praeoral region is large and possesses a well-developed 

 prototroch and an apical plate. ]!Sear the apical plate 

 there appear two small ciliated elevations each consisting 

 of a single cell with cilia. The shell gland secretes a 

 shell which becomes more and more convex outwards, 

 attaining a. rounded cup-shape. The originally solid 

 mass of endoderm becomes hollowed out and arranges 

 itself as the lining of the midgut. It gives off a diver- 

 ticulum postero-veutrally, which ultimately opens to the 

 exterior forming the rectum. The fore-gut gives ofl: a 

 diverticulum ventrally, which is the rudiment of the 

 radular sac. 



The ectoderm thickens and projects along a band just 

 external to the shell margin, and this is the first trace 

 of the mantle skirt. The anal diverticulum reaches the 

 ectoderm ventral to this, and between anus and mouth 

 we now find the well-marked pair of foot rudiments. 

 A pair of depressions occur at the sides of the mouth 

 and grow inwards along the sides of the foot, becoming 

 closed sacs and forming the otocysts. 



As the shell and foot continue to grow, the anal diver- 

 ticulum and the rudiment of the brant hial cavity become 

 confined between them, and we then observe their 

 migration from the postero-ventral position, along the 

 animal's right side, ventral to the mantle skirt, up to au 



