ALIMENTARY CANAL. 677 



junction of the oesophagus with the broad intestine (PL XXXII. f. 5) we 

 find a cluster of diverticula, in the form of pointed pouches. Seen from the 

 actinal side, a simple mesentery unites the interior edges of the alimentary 

 canal (PL XXXII. f. 3). The outer edges of the canal, which follow the 

 edge of the test, are attached by delicate mesenteries ; their course can be 

 traced on the interior of the test (PL XXXII. f. 2; PL XXXIII. f. 2). 



In the Petalosticha the trend and course of the alimentary canal appear to 

 vary greatly, and may prove of value as systematic characters. In Spatan- 

 gus (PL XXXII. f. it, IS, copied from Hoffman) the commencement of the 

 alimentary canal runs in the posterior interambulacral space, rising slightly 

 towards the apical system, trending to the right ; it runs then from right to 

 left, rising very gradually until it has reached the anterior extremity of the 

 test, where a huge diverticulum, trending upwards and towards the posterior 

 extremity, arises. It continues to run parallel to the test, forming a nearly 

 complete circuit, till it nearly reaches the anterior extremity. It then curves 

 sharply upwards, running backwards from right to left towards the posterior 

 extremity, completing somewhat more than a half-circuit ; it then forms a 

 sharp curve towards the central part of the cavity, becoming much wider at 

 the same time, and, when it has reached the longitudinal axis, suddenly nar- 

 rows and extends to the anus in the posterior interambulacral space, immedi- 

 ately above the place of its origin, only in the abactinal part of the test. We 

 can distinguish, as in the Desmosticha and Clypeastroids, an oesophagus, a 

 broad part of the alimentary canal, and a terminal anal extremity. The ali- 

 mentary canal is attached to the test by innumerable minute mesenteries. 

 Distinct traces of the course of this attachment can still be seen (PL 

 XXXIV. f. 2) on the interior of the abactinal part of the test, as in the 

 regular Echini ; but in addition to these we have large mesenteries which 

 connect the central parts of the alimentary canal, and are not found either 

 in the Desmosticha or in the Clypeastroids. 



In Metalia the trend of the oesophagus follows the line of the posterior 

 interambulacral field (PL XXXII. f. 11). It consists at first of a large tri- 

 angular actinal pouch, passing gradually into a narrow oesophagus. At the 

 junction of the oesophagus with the alimentary canal proper is found a 

 cluster of small diverticula resembling those of Rhynchopygus, and not a 

 single large diverticulum as in Spatangus proper. 



The mesenteric plates holding the alimentary canal in place are more 

 complicated in Spatangus than in Rhynchopygus, where the only prominent 



