VIII ECHINODERMATA— MORPHOLOGY OF SKELETON 337 



certain pieces of the calcareous ring of the Holothurioidea corre- 

 spond with the orals of other Ecliinoderms cannot at present be 

 determined. 



C. The Perisomatic Skeleton. ^ 



All those skeletal pieces which protect the body, between the apical 

 and the oral systems, taken together, form the perisomatic skeleton 

 of the Echinodermata. It is obvious that the extent of the periso- 

 matic skeleton must vary inversely with that of the polar (apical and 

 oral) systems, ^^'here the polar systems form only a small part of 

 the body wall the perisomatic skeleton is the more strongly developed, 

 and vice versd. In the Blastoidea, for ^ ^ 



example, nearly the whole of the test 

 is formed by the polar systems 

 (especially the apical), while in most 

 Echinoidea, Asteroklea, and Oplduroidea, 

 the perisomatic system covers nearly 

 the whole body. AVhere the equatorial 

 zone of the body is produced into 

 variously shaped branched or un- 

 branched arms, as in most Pelmatozoa, 

 Asteroidea, and Ophiuroidea, the skeleton 

 of these arms is exclusively formed by 

 perisomatic pieces. It is at present 

 impossible to prove any definite 

 homologies between the parts of the 

 perisomatic systems throughout the 

 Echinodermata. 





In the cutis of the Holothurioidea, 



I. Holothurioidea. Fio. 300. -Microscopic calcareous bodies 



of Holothurioidea. 1, Auchor ;ind anchor 

 plate of Synapta inhserens, O. F. M. ; 2, 

 as well in the body wall as in the "stool"of Cucumaria longipeda, Semp; 3, 



wall of the tentacles, ambulacra, tube- ^™"fo"« ^fj °f cucumaria crucifera 



' ' Semp; 4, rod from one of the tube-feet of 



feet, and ambulacral papillae, there are sticopus japonicus ; 5, supporting piate 



found enormous numbers of micro- from one of the tube-feet of Stychopus 

 SCOpically minute calcareous bodies of XTy"! r. ro'd froTthe' veltrfKambull- 

 definite shapes (Fig. 300). These give oral appendages of Oneirophanta mutabilis, 

 the integument a firm and rough Theel ; S, latticed hemisphere of Colochirus 

 . , mi • • • 1 • • f cucumis, Semp; 9, "wheel" of Acantho- 



consistency. Their principal Slgnifi- trochus miraWlis, Dan. and Kor. 



cance may well be that of protection. 



These small calcareous bodies may be called, according to their shapes, 

 "anchors," "wheels," "rods," "anchor plates," "crosses," "lattices," 

 "stools," "buckles," "biscuits," "cups," "rosettes," etc. 



^ Ou the aiitlior's use of the term " perisomatic," see footnote, p. 362. 

 VOL. II Z 



