212 COMPARATIVE ANATOMY. 



or two new vesicles are formed from the sides of the enteric caecal 

 tube. In this way three different bodies are differentiated from the 

 enteron. The two-paired vesicles, which lie at the sides of the enteron, 

 represent the commencement of the ccelom; the third vesicle 

 becomes connected with the dorsal ectoderm and opens on to it ; this 

 is the commencement of the water- vascular system. This apparatus, 

 like the lining cell-layer of the ccelom, takes its origin therefore 

 from the enteron, and from that portion of it, which is without 

 doubt its hinder part, although it appears first of all and grows 

 inwards, from what is later on the anus. This arrangement appears to 

 indicate that there are arrangements in the water-vascular system, as 

 well as in the ccelom (for the two are connected together), which are 

 phylogenetically connected with the terminal division of the enteron ; 

 in this case this tract of the enteron is not homologous with the 

 enteron of a Gastrula, but corresponds a priori to a hind-gut, the 

 early development of which is clearly due to the complicated 

 character of the organs about to be differentiated from it. These 

 orgaus are those which are especially necessary to the organism. I 

 consider therefore that the first formed rudiment of the enteron is 

 not a Gastrasa-enteron, and that its orifice is not the primitive mouth, 

 but that they are respectively the true hind-gut and anus. The 

 median division of the enteron which is divided from the hind-gut 

 is morphologically a part of it. The differentiation of the above- 

 mentioned organs out of the hind-gut points to stages in which 

 organs were connected with the hind-gut in much the same way as 

 they are in many Gephyrea. But as yet it is impossible to prove 

 directly that such structures have been passed on to the Echino- 

 derma ; and it is better to regard these remarkable processes as 

 presenting us with a problem which has still to be solved. 



When the body of the Echinoderm is formed in and partly 

 from the larva, the enteron of the larva does not completely pass 

 into it. The perisome, when formed, first grows round its middle 

 part, aud in the sea-stars takes up this only with the hind-gut. In 

 the Echinoida the anus also appears to be formed anew. The larval 

 enteron is retained most completely in the mature stage of the 

 Holothuroi'da. 



The fully-developed enteron is found to hang, in the mature 

 Echinoderm, in a ccelom, which is often wide, and undergoes 

 various changes during its differentiation, which are generally corre- 

 lated with the characters of the perisome. As a rule the mouth 

 always retains its position in the middle of the ventral surface of the 

 body. 



§ 172. 



The mouth in the sea-stars has a radiate form, owing to the 

 projection into it of interradial processes ; hard papillas and spicules 

 are formed by the perisome, and function as masticatory organs. 

 They are specially developed in the Ophiurida, where they generally 



