126 



GASTKOPODA. 



~ nua. 



through a rearrangement of the cells which also increase greatly in 

 number (Fig. 52). From the posterior end, where the mesoderm- 

 cells lie, two very regular mesoderm-bands grow out (Fig. 52). The 

 shell-gland appears dorsally before this stage as a depression formed 

 of columnar ectoderm-cells ; over this gland, the shell-integument is 

 secreted later. 



Patten asserts that the foot arises at a very early stage in a 

 remarkable manner. It is said to be produced from two prominences 

 which lie ventrally at the posterior end of the body (Fig. 51 A). These 



flank the blastopore on 

 either side at a time when 

 the latter still is a round 

 aperture. As soon as it is 

 displaced anteriorly, they 

 shift together and unite to 

 form the foot, the double 

 origin of which can be 

 recognised even in later 

 stages through the presence 

 of a median groove. 



Up to this stage, the 

 pre-oral region was speci- 

 ally large and bell-shaped 

 (Fig. 51). It is Separated 

 from the posterior section 

 by the pre-oral ciliated 

 ring, which is composed 

 of three rows of cells, the 

 middle row being provided with the strongest cilia (Fig. 52). A tuft 

 of long cilia appears on the apical plate, and near it lie two promi- 

 nences bearing stiff cilia (Fig. 51 B). These would recall the cephalic 

 tentacles of the Annelida did not each of them consist of a single cell. 

 As development advances, the pre-oral part flattens out considerably, 

 and the apical plate, which has already appeared as a median 

 thickening (Fig. 52, s), now takes up a considerable part of the pre- 

 oral section (Fig. 53, sp). At the posterior end of the larva also, a 

 tuft of long cilia can be seen ; these belong to the anal cells above- 

 mentioned. The shell-gland which was previously invaginated has 

 now flattened out, and the dorsal surface even appears convex. The 

 epithelium, which was formerly very thick in this region, now consists 

 merely of flattened cells (Fig. 53). The shell itself has become cup- 



Fig. 52. — Horizontal section of an older larva 

 of Patella (after Patten). a, ciliated anal 

 cells ; md, enteron ; mes, mesoderm ; s, apical 

 plate ; w, ciliated ring. 



