ECHINOIDEA 



261 



somewhat different iii structure in the various groups, and there- 

 fore of considerable systematic importance. P]ach tooth is enclosed 

 by two larger pieces, which form together a " pyramid " ; on 

 their upper end are found a pair of smaller pieces, the " epiphyses ", 

 which may join in the mid-line and thus form an arch over the 

 pyramid (the " camarodont " type, Fig. 146, 1), or they may be 

 shorter so as not to join in the mid-line (the " stirodont " type. 

 Fig. 146, 2). Above the interspace between each two adjoining 

 pyramids lies an unpaired piece, the " rotula ", and above this 

 again a slender, arched piece, the " compass ", the outer end of 

 which is usually heart-shaped. To the pyramids are attached 

 muscles for protruding and retracting the dental apparatus ; their 

 other end is attached to the 

 "auricles ", internal processes 

 from the test at the peri- 

 stomial edge. The pyramids 

 are connected with each other 

 by short but powerful ' ' inter- 

 pyramidal " muscles, the 

 masticatory muscles proper. 

 In addition there is a pair 

 of muscles from the outer 

 end of the compass, and a 

 muscle which connects all the 

 five compasses, thus forming 

 a pentagon on the upj^er side 

 of the dental apparatus. The 

 teeth, which consist of a hard, enamel-like substance, are 

 constantly growing at their upper ends, and are pushed out 

 steadily as the point wears off. They may be concave or 

 have a longitudinal keel on the inner side. All Spatangoids 

 lack teeth and tooth apparatus completely. In some Cassidulids 

 teeth have been found to exist in the young ; they are, however, 

 soon resorbed, and the adult form lacks teeth completely. 



The intestinal canal is a simple tube, attached to the test by 

 means of strong mesenteries ; it runs first nearly all round along 

 the test, then bends directly back on itself, finally opening through 

 the anus. Thus it does not form a real spiral coil. In the irreg- 

 ular Echinoids there is a large blind gut issuing from the intestine, 

 and a small, thin " siphon " running close along the intestine and 

 opening into it at both ends. A pair of blood-vessels accompany 

 the intestinal tube, being connected with a network of blood- 

 sinuses in its walls. 



Fig. 146. — Dental apparatus of regular 

 Echinoids. x 3. (After Jackson ; 

 from Danniark's Fauna.) 



1. Of Strongylocentr. drobachiensis (cama- 

 rodont type). 2. A single pyramid of the dental 

 apparatus of Arbacia (stirodont type), c, Com- 

 pass ; ep, Epiphysis ; m, Interpyramidal 

 muscle ; p, Pyramid ; r, Eotula ; t, Tooth. 



