CLASS I 



ECHINOIDEA 



267 



Fio. 374. 



.4, Clypeaster retkulatus Loven. 



Recent. The dental system entire, seen from 

 above. The braces are placed upon the sutures of adjoining pyramids, with an 

 epiphysis on either side. Teeth in line with the mesial sutures of half-pyramids, 

 an<l within the ring formed by the supra-alveolar crests (after Loven). B, a, Front 

 view of a single pyramid ; 6, side view of one of the half-pyramids. 



muscles (Fig. 372, A) which contract the jaws; ten internal and ten external 

 brace muscles, which are tiny and extend from the brace to the epiphyses ; 

 five circular compass muscles which dorsally connect the compasses ; ten radial 

 compass muscles 

 which extend 

 from the com- 

 pass to the base 

 of the test. 

 These last are 

 wanting in Cly- 

 peastroids as in 

 that group com- 

 passes are ab- 

 sent. 



Perignathic 

 Girdle. — Cer- 

 tain muscles of 



the lantern, namely, the retractor, protractor and radial compass, pass to and 

 are inserted on the base of the test, and certain important processes, the 

 perignathic girdle, may be built for insertion of these muscles. Loven showed 

 that in young Goniocidaris the lantern muscles are all attached directly on the 

 base of the interam^^ulacra, and the same method probably existed in the 

 Perischoechinoida, as in that order no perignathic processes have been seen. 

 In adult Cidaroida elevated processes, the apophyses exist as strong internal 

 upgrowths of the ventral border of the basicoronal interambulacral plates ; to 

 these apophyses in this order alone are attached the retractor, protractor and 

 radial compass muscles. In the order Centrechinoida the apophyses or up- 

 growths of the interambulacral plates may be feebly or strongly developed, and 

 to them are attached the protractor and radial compass muscles. In this order 

 a new structure appears consisting of two separate calcareous plates, the 

 auricles, which are united by close suture to the basicoronal ambulacral plates. 

 The auricles exist as separate styles or in some genera in development may 

 grow into large plates which arch and join in suture over the ambulacral area. 

 Auricles give insertion to the retractor muscles, which combination of 

 apophyses and auricles is known in this order only. In the Holectypoids 

 low apophyses and auricles, or auricles alone may exist ; as this groui? 

 occurs fossil only, the muscles can only be inferred. In Clypeastroids 

 apophyses are wanting, but low or high aui'icles exist on the ambulacral 

 plates, or even may be transferred to the basicoronal interambulacral plates 

 as seen in Echinarachnius. In Clypeastroids retractor and protractor muscles 

 are both inserted on the auricles, a condition peculiar to the group. 



Tubercles and Spines. — The plates of Echini bear more or less numerous 

 tubercles and granules of various sizes which bear larger or smaller spines. 

 The larger and completely developed tubercles are called primaries ; those 

 of a smaller size are secondaries; and very small tubercles, sometimes 

 incomplete in their development, are miliaries. Grannies are irregular or 

 nodular projections of the test ; they may be large and widely separated, 

 or very numerous and of various sizes. The base of a tubercle is termed 

 the boss, and its upper part may be either plain or crenulated. The boss 

 supports a rounded mamelon, which is said to be perforated when pierced by a 



