214 COMPAEATIVE ANATOMY. 



may be also seen in young Asteroi'da ; in them it is only seen for a 

 time during their development^ but in the Orinoida it is continued 

 as a permanent condition. 



The enteron is attached to the body-wall by radial fibres. The 

 radial caeca of the Asteroida are attached to their body-wall by a 

 special peritoneal fold, which extends along each caecum. 



§ 173. 



In the Echinoida the mouth is similarly provided with Mastica- 

 tory Organs, but they are removed from the outer surface and placed 

 in the coelom. They there form an apparatus, which, in the Clypea- 

 strida, consists of five pairs of triangular calcareous pieces, but in the 

 Echinothurida, Cidarida, and Echinida is much more complicated. 

 Five pieces directed towards one another carry a tooth-like point, 

 and are united with several others into a complex organ known as 

 the "Lantern of Aristotle;" the oesophagus traverses it. The 

 entei-ic tube always describes several coils. The narrow fore-gut 

 passes into a wider portion, which forms the longest part of the canal. 

 It has sometimes faintly -indicated diverticula (Echinida), sometimes 

 veritable caeca (Clypeastrida), which (as in Laganum) project into 

 the cavity of body which is marked off by the supporting pillars 

 of the calcareous shell. In these forms " mesenteric fibres " extend 

 to the body-wall for the whole length of the coiled intestine. 



In the Holothuroida the enteric tube, which is longer than the 

 body, forms a double loop, while in the Synaptte (with the exception 

 of the Chirodotae) it extends straight through the body-cavity, and 

 is provided with numerous diverticula. A muscular portion of the 

 enteron which succeeds the oesophagus is to be regarded as a special 

 differentiation; it appears to function as a muscular stomach 

 (Synaptae). This character is also seen in the Asteroi'da, where the 

 oesophagus has in the same way a stronger muscular wall than the 

 rest of the intestine. The portion of the intestine behind the 

 muscular part in the Holothuroida may thus correspond to the 

 stomach of the Asteroida. The end of the canal is widened out in 

 the Holothuroida ; bu.t this only corresponds to the hind-gut of the 

 Asteroida, although it is called a cloaca; it has two or more arbores- 

 cent organs opening into it. 



A sieve-like fenestrated lamella fastens the canal to the body- 

 wall. This mesentery is simpler in the Synaptge which have a 

 straight canal, while in Chirodota it is separated into three parts, in 

 correspondence with the extent of the enteric loop ; each part is 

 connected with an interradial portion of the body-wall. 



