266 



ECHINODERMATA— ECHINOZOA 



PHYLÜM IV 



B 



Holectypina, Clypeastrina). Pyramids, each composed of two halyes, are 

 roughly triangulär in section, present a median suture, above which is a 



shallow or a deep 

 open Space, the 

 foramen magnum. 

 On the inner face 

 the pyramid Sup- 

 ports and embraces 

 the tooth, and 

 laterally in regulär 

 Echini has ridges 

 for the attachment 

 of interpyramidal 

 Fig. 372. muscles. The 



Dental apparatus of the Recent Strongylocentrotus dröbachiensis (O, F. Müller). UDDer faCC of the 



A, Lantern showing teeth, pyramids, interpyramidal muscles, styloid processes of ^^ . , 



dental slides, epiphyses, crests and compasses. B, Pyramid showing on one side pyramiu, aS Seen 



protractor and retractor muscles, epiphysis removed on left side. C, top of ^.'Up« ^-'Up p-nmhv 



lantern, at areas 2, 3 a whole tooth in place ; at 1 pulj)y i)art of tooth removed ; at Wneil lim epipuy- 



4 tooth removed. At area III compass, brace and epiphyses in place ; at V com- gjg jg remOVed is 



pass removed ; at IV brace also removed ; at I the epiphysis of one side is removed ' 



to show pits in top of pyramid. 2/^ (after Jackson). a smOOth IlOOr 



(Paleozoic genera, 

 Cidaroida) ; or is pitted (Centrechinoida). An epiphysis caps each half- 

 pyramid, to which it is joined by close suture, it presents a glenoid 

 cavity and tubercles for interlocking with the 

 brace. The epiphyses are narrow in all Echini 

 except the Camarodonta in which they are wide 

 and meet in suture over the foramen magnum ; 

 here also they bear crests which support the teeth 

 dorsally. The brace is a block-shaped plate which 

 rests on and interlocks with the epiphyses. The 

 compass rests on the brace and consists of an inner 

 and an outer part, joined by suture; the outer 

 part is usually bifid but may be rounded. The 

 angle of 'inclination of the lantern, the teeth 



grooved or keeled, the depth of the foramen mag- py,^^id „f Recent strongyiocen- 

 num, absence or presence of pits in the top of the ^™^«*' dröbacUensis. a, in sid« view 



., ^ ^ .^ . 11« showing corrugations for attachment 



pyramids, and narrow or wide epiphyses and their of interpyramidal muscie, epiphysis 



.^^^-,- .,• , • i. 2. £ j. •!• with crest, glenoid cavity, external 



meetmg in suture are important f eatures in classi- and internal tubercles. b; Pyramid 



fication from centre showing dental slide and 



* , other parts as in A (after Jackson). 



The ]aws of the Holectypoids are similar to 

 those just described, but in the Clypeastroids they are low, often asym- 

 metrical, and the teeth are aslant or even nearly horizontal (Fig. 374). 

 Compasses are absent, and the braces are rudimentary. The pyramids are 

 solid almost to their upper part, more or less concave, or re-entering on the 

 outer side, and are not always of the same size in each area. Jaws of 

 Echini are rarely preserved in the fossil State. 



The muscles of the lantern are numerous and complex, and their insertion 

 in the test is of systematic importance. There are sixty lantern muscles in 

 regulär Echini which in brief are, ten protractors inserted on the outer face 

 of the pyramids and base of the test ; they extend the lantern ; ten retractors 

 similar ly situated which open the jaws (Fig. 372, B) ; five interpyramidal 



Fig. 373. 



