ALIMENTARY CANAL OF ECHINODERMA. 



215 



Appendages of the Alimentary Canal. 



174. 



*n? . 



The above-mentioned radial casca of the Asteroi'da might be 

 regarded as organs differentiated from the primitive enteron, were 

 it not that they must be regarded differently from a phylogenetic 

 point of view. I consider 

 that only certain other 

 interradial casca are to 

 be regarded as appen- 

 dages of this kind ; they 

 present very various de- 

 grees of development. In 

 the aproctous Asterida 

 they are absent, or are re- 

 duced to two (Astropec- 

 ten), while in the others 

 they are often very greatly 

 developed. Archaster has 

 five such csecal sacs, divided 

 at their ends, and in Oul- 

 cita this division is carried 

 still further, so that each 

 branch forms a racemose 

 tube. In this way these 

 appendages acquire the 

 form of glands, and ex- 

 hibit relationship with a 

 structure which is very 

 common in the Holothu- 

 roi'da. 



The structure in ques- 

 tion is connected with the 

 terminal portion of the 

 alimentary canal, known 

 as the " cloaca," and 

 as a rule consists of two 

 chief trunks, with short 

 branches, which extend 

 forwards throughout the whole length of the body-cavity (Fig. 109, 

 ?•), and are provided with a large number of ramified caacal tubes. 

 Although the function of these organs, which were formerly known 

 as " lungs,"* and considered to be internal respiratory organs, is 

 different from that of the interradial caecal tubes of the enteron of 

 the Asterida, yet they are exactly the same morphologically, and 

 are a further development of the simpler tubes of the Asterida. 



Fig. 109. Enteric canal and tree-like organs of 

 aHolothurian. Mouth, i Enteric tube. 

 d Cloaca. a Anus. c Branched stone-canal. 

 £> Polian vesicle, rr Tree-like organs, r' Connec- 

 tion between them, at the opening into the cloaca. 

 m Longitudinal muscular layer of the body. 



