574 ALIMENTARY CANAL. 



We can distinguish in the walls of the alimentary canal an outer and inner 

 connecting layer, enclosing a transverse and a longitudinal muscular layer. 



Although at first sight the general course of the alimentary canal appears 

 totally different in the Clypeastroids and in the Petalostieha from that of the 

 Desmosticha, we do not find on ultimate analysis such a wide difference. 



While examining the course of the alimentary canal from the point at 

 which it leaves the pyramid in the Desmosticha, it was placed in such a way 

 that, when seen from above, the first winding should be from left to right 

 along the test. This does not place the different genera in a homologous 

 position, as in one case we may have the first loop of the alimentary canal so 

 placed as to be nearest what in one genus would he the left anterior ambu- 

 lacrum, and in the other the left posterior ambulacrum, if we determine the-,' 

 ambulacra by the position of the madreporic genital ; in all the genera 1 have 

 examined the alimentary canal trends from the pyramid to the interambula- 

 cral space in which the madreporic body is placed. On determining whether 

 this has a definite relation to the anal opening, we find that it is placed nearer 

 a trivium, hut not the same trivium. in different genera. If we now take the 

 trend of the terminal intestine as our guide, we find that although the rela- 

 tive direction of the terminal part of the alimentary canal and of the origin 

 does not correspond with that of the irregular Echini, yet. in the Desmos- 

 ticha, if we place one of the ambulacra in the prolongation of the direc- 

 tion of the anal termination of the alimentary canal, the madreporic body 

 will always be in the left posterior Lnterambulacral space. The above posi- 

 tion of the Desmosticha. with the madreporic body in the left posterior 

 interambulacral space, is the only one properly homologous to that of the 

 Clypeastroids and Spatangoids. in which we always find the odd ambulacrum 

 in the prolongation of the trend of the terminal anal pari of the alimentary 

 canal. 



In Clypeastroids the pyramidal oesophagus is quite slender ; it opens into 

 a Hat triangular pouch, the broad oesophagus which runs in the trend of the 

 longitudinal axis (Ph. XXVIII., XXIX. lower figs. ; PL XXX. upper' Jig.) ; at 

 its junction with the alimentary canal proper it turns sharply round to the 

 left, and runs back along the edge of the test, past the posterior extremity, 

 completely round the ambitus to the anterior L't\<i:c. where it forms a second 

 loop, and runs back again on the inside of the outer part of the alimentary 

 canal to the anal opening (PL XXXVIII. f. l, 2, XXIX. loiver Jiy. ; PL 

 XXX. upper Jig.). 



