ALIMENTAEY CAN"AL OF ECHINODEEIMA. 211 



As tliis apparatus lies on the terminal ganglionic swelling of the 

 radial nerves, and the cells give off fine processes to this ganglion, 

 the two parts may be regarded as connected at this point (Astera- 

 canthion rubens). Each eye which consists of a complex of cells 

 is a differentiation of the epithelial layer, and resembles therefore 

 the optic organs of other Invertebrata. 



Alimentary Canal. 



§ 171. 



The alimentary canal, which varies greatly in character in the 

 adult Echinoderma, has a simpler predecessor in the primitive 

 enteron of the larval form, which is similar in all Echinoderma. Of 

 course this does not refer to those forms in which there is no larval 

 stage, and where the development is compressed. 



The first rudiment of the enteron is formed by the in-growth of 

 the cell-layer, which invests the body of the young larva. This gives 

 rise to a ctecal tube, which is pushed down into the body, and the wall 

 of this tube forms the endoderm, while the outer cell-layer represents 

 the ectoderm. The organism is in fact a Gastrula. The entrance 

 into the rudimentary enteron is regarded as the primitive mouth. 

 A second invagination soon grows from the other side of the body 

 towards the blind end of the enteron ; this unites with the enteron, 

 becomes hollow, and so forms a continuous tube with the part first 

 formed. The parts formed last ara the mouth and the oesophagus, 

 which is connected with it, and the part formed first is the mid- and 

 the hind-gut. The orifice which becomes later the anus, and the 

 portion of the enteron connected with it, are consequently the parts 

 first formed. 



The larval intestinal canal is formed of three portions (cf. Fig. 

 94, A B). A wide oral opening leads into a contractile tube lying 

 in the long axis of the body ; this is the pharynx or oesophagus. 

 Then follows a wider part, the mid-gut or stomach, which is con- 

 tinued into a narrow and retort-shaped tube, which is the hind-gut 

 and leads to the anus. These three portions correspond exactly to 

 the primitive divisions of the canal, which are distinguishable in 

 nearly all Vermes. The mouth and anus are at first on different 

 surfaces of the body of the larva. As the body is differentiated, 

 especially by the development of the ciliated band, they apparently 

 come to lie on one and the same surface, the so-called anterior side. 

 It is, however, quite clear that the ciliated baud distinctly divides 

 two surfaces of the body; a decreased oral, and an increased anal 

 surface turned towards the former. 



But before the enteron is fuUy developed by becoming connected 

 with the fore-gut, a portion of it, which forms a closed vesicle, is 

 constricted off. Two pieces are then separated off from this vesicle, 



p 2 



