366 



( 'oMPARATIVE AN ATOM Y 



CHAP. 



almost centrally (Fig. 317). In consequence of this shifting the 

 ambulacra are, of course, very unequal in length. 



The (palaeozoic) Camcrataare distinguished l>y the tendency to strong development 

 of the perifiomatic skeleton in the calyx, and by the plates being so linnly inter- 

 connected as to form a rigid test. In the formation of the dorsal cup. the 

 bases of the arms are incorporated to a certain extent in such a manner that their 

 lower brachials become fixed plates of the cup. In the live interradii of the dorsal 

 cup, interradials appear, to which, in the posterior interradius. special anal 

 plates are often added. In those cases in which the arms take part in the formation 

 of the capsule beyond their first branchings, interdistichals. etc., may connect the 

 branches firmly together. 



The tegmen calycis also consists of plates which are usually very numerous and 

 firmly connected together. Just as the mouth is always covered by characteristically 

 arranged, closely fitting, orals, so also the ambulacra! furrows arc never open, but are 

 always arched over by large covering plates, some of which may be distinguished by 



A B 



Ci 



FIG. Sis. Actinometra (after P. H. Carpenter), bin-rums to illustrate the course^ of the lb<l 

 grooves over the teamen calyds. Aj-Ej, the directions of the five pairs of arms. In the centre the 

 anal tube. 







their greater size. In the older forms, the tegmen is, as a rule, rather fiat, and the 

 covering plates of the ambulacral skeleton appear at the surface. In the course of 

 the geological development of the palaeozoic epoch, however, the tegmen bulges out 

 more and more, and finally forms a high, firmly plated "vault" or dome (Figs. 

 253, 254, pp. 307. 308), which, immediately behind its centre, may be prolonged 

 to form a tube, often of greater length than the arms, with the anus at its tip. 

 Where such a highly arched dome is developed, the interambulacral plates, which 

 border the ambulacral furrows, send out processes over the latter. The processes 

 (which are closely joined to one another) from one side of the ambulacra meet and 

 become firmly connected with those from the other, so that the ambulacral furrows 

 with their skeletons are completely arched over, and are not externally visible. 



(This condition was until quite recently wrongly explained as follows. The 

 Camemta possessed an inner, naked, or merely loosely plated tegmen. in which the 

 ambulacra ran from the mouth in the centre to the periphery, and this tegmen 

 was arched over by a firmly plated vault in such a way, that between the tegmen 

 and the vault there was a free space. ) 



The interradial plates of the tegmen are often continued directly. >'.< . without a 

 boundary line, into the interradial plates of the dorsal cup. 



The anus, surrounded by special plates, lies in the posterior interradius. 



