xvi ECHINODEEMATA 477 



The larval oesophagus or stomodaeum, as iu Asterias, becomes 

 disconnected from the larval stomach, shallows out and disappears. 

 The larval stomach, which, as we have seen, forms the adult pyloric 

 sac, hegins to give off blunt outgrowths into the cavities of the 

 nascent arms : these are the rudiments of the pyloric caeca 

 (Fig. 369). The adult stomach, begun as we have noted as an out- 

 growth on the left side of the larval stomach, increases in size and 

 comes in contact with the ectoderm at a spot between the dorsal and 

 ventral horns of the left posterior coelom. These horns meet above 

 it, and so the left posterior coelom is converted into a ring. Within 

 this ring lies the ring formed by the hydrocoele, and beneath this the 

 ring formed by the peri-oral coelom. The holes in the septum 

 dividing the anterior coelom from the left posterior coelom become 

 healed up, and with the progressive constriction of the neck of the 



II 



FIG. 367. Views from the side of a larva of Asta-ina gilihusa seven clays old in the initial 

 stages of metamorphosis. .'After Ludwig.) 



A, from the right side. B, from the left side. 1-5, lobes of hydrocoele. I-V, rudiments of arms ; 



in-.l, juae-oral lobe. 



prae-oral lobe the anterior coelom becomes divided into a transitory 

 portion, situated in the stalk, and a permanent portion, the axial 

 sinus, which is included within the disc of the star-fish. The hydro- 

 coele communicates with the anterior coelom not only through the 

 stone-canal, which has become a closed tube, but through an opening 

 in the neighbourhood of its third lobe which does not become closed 

 until metamorphoses is nearly complete. 



The adult nervous system of Asterina arises as a plexus of 

 ganglion cells and fibres, beneath the ectoderm which overlies the 

 perihaeuial spaces and the lobes of the hydrocoele. Goto states 

 that, in Asterias, part of this ectoderm is derived from the longi- 

 tudinal ciliated band of the larva. If this statement could be confirmed 

 it would be a matter of great interest. 



Of the development of the adult calcareous skeleton of Asterina 

 we have a full account from Ludwig (1882), and we have also some 

 information about the origin of the calcareous skeleton in Asterias 

 rubens from Bury (1895). The first traces of calcareous plates in 



