576 INVERTEBRATE MORPHOLOGY. 



traversed in some forms by the calcareous pillars already 

 mentioned, as well as by a perforated mesentery extending 

 from the inner surface of the test to the intestine and follow- 

 ing the convolutions of the latter. In the bilateral Echinoids, 

 which as a rule swallow large quantities of sand, the mesen- 

 tery is much stronger than in other forms, and additional 

 mesenterial bands are added to assist in the support of the 

 intestine. As in the Ophiuroids, a circular partition extends 

 from the oesophagus outwards to be inserted into the test in 

 the neighborhood of the auriculae, enclosing a peripharyngeal 

 space (Fig. 265, pph), which has no communication with the 

 rest of the ccelom, and contains the organs of mastication when 

 these are present. In many forms the partition is pouched 

 out into five radial diverticula which project into the general 

 coelom and are known as the organs of Stewart or as internal 

 branchiae. In those radial Echinoids in which these structures 

 are absent, ten lobed diverticula of the floor of the peripharyn- 

 geal space project upon the outside of the body at the margin 

 of the peristome, a pair being situated in each interambula- 

 cral region ; these are termed external branchise (hr). At the 

 aboral surface of the body in the radial forms a partition 

 similar to that enclosing the peripharyngeal space is found, 

 surrounding the terminal portion of the intestine and enclos- 

 ing a subperiproctal cavity {pp), while within this occurs a 

 second partition shutting off a perianal space {pa). Muscular 

 fibres occur in these partitions, and it has been suggested that 

 by contracting and thus compressing the fluid contained in 

 the spaces they serve to close the lumen of the rectum and 

 the anus. 



As regards the axial sinus {as) the Echinoids resemble the 

 Asteroids and Ophiuroids in that the portion of the ccelom 

 into which the larval stone-canal opens persists in the adult 

 and forms a pouch extending downwards towards the oral 

 surface parallel to the stone-canal, the ovoid gland {og) devel- 

 oping in its walls. Into the upper portion of it the adult 

 stone-canal (sc) opens, and it communicates with the exterior 

 through the madreporiform tubercle (71/). 



The so-called blood-vessels are, as in the Ophiuroidea, 

 portions of the lacunar system, and are contained in peri- 



