THE RISE OF THE LARVA, ETC. 



127 



shaped. The somewhat swollen edge which is seen bordering the shell 

 represents the margin of the mantle, the mantle itself being covered 

 by the thin horny shell. The enteron has considerably widened and 

 is now sac-like and, connected with it posteriorly, a pointed appendage 

 can be seen ; this unites later with the ectoderm to form the anus. 

 In the stomodaeum, which has now enlarged, an outgrowth (r) is 

 visible ; this is the rudiment of the radular sac which was found to 

 appear in an exactly 

 similar way in the 

 Amphineura and the 

 Scaphopoda. 



On each side of the 

 mouth, right and left, 

 a depression appears 

 even at an earlier 

 stage ; this deepens to 

 form a vesicle, which 

 finally becomes sepa- 

 rated from the ecto- 

 derm. The two vesicles 

 thus formed are the 

 otocysts. They lie at 

 the base of the foot, 

 which is commencing 

 to develop into a large 

 prominence and in 

 which there is a rich 

 accumulation of meso- 

 derm-cells. The meso- 



derm has lost its regular arrangement, single cells becoming detached 

 from the mesoderm-bands, and being distributed in the primary 

 body-cavity ; these no doubt represent the rudiment of the covering 

 of the ectodermal and entodermal organs already formed. Certain 

 of these cells elongate and give rise to muscle-fibres, a number of 

 which become attached to a point on the dorsal surface, where they 

 finally become firmly connected with the shell, and yield the retractor 

 muscle by means of which the larval body can be withdrawn into 

 the shell, as soon as the latter has attained the proper size. 



In the stages depicted in Figs. 51 arid 52, and even in the later 

 condition, a median section of which is given in Fig. 53, the Patella 

 larva closely resembles the Troehophore stage met with in the 



md 



FIG. 53. Median longitudinal section through the 

 larva of Patella in the later Troehophore stage (after 

 PATTEN), a, the ciliated (anal) cells at the posterior 

 end ; /, foot ; m, mouth ; md, enteron ; mes, meso- 

 derm ; r, radular sac ; s, shell ; sp, apical plate. 



