vin ECHINODERMATA MORPHOLOGY OF SKELETON 337 



certain pieces of the calcareous ring of the Holothurioidea corre- 

 spond with the orals of other Echinoderms cannot at present be 

 determined. 



C. The Perisomatie Skeleton. 1 



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

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

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

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

 oral) systems. AVhere the polar systems form only a small part of 

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

 and rice versa. In the Blastoidea, for 

 example, nearly the whole of the test 

 is formed by the polar systems 

 (especially the apical), while in most 

 Echinoidea, Asteroidea, and Ophiuroidea, 

 the perisomatie system covers nearly 

 the whole body. Where the equatorial 

 zone of the body is produced into 

 variously shaped branched or un- 

 branched arms, as in most Pelmatozoa, 

 Asteroidea, and OpMuroidcu, the skeleton 

 of these arms is exclusively formed by 

 perisomatie pieces. It is at present 

 impossible to prove any definite 

 homologies between the parts of the 

 perisomatie systems throughout the 

 Echinodermata. 



I. Holothurioidea. 



as 



FIG. 300. Microscopic calcareous bodies 

 of Holothurioidea. 1, Anchor and anchor 

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

 "stool" of Cucumaria longipeda, Semp ; 3, 



In the cutis of the Holothurioidea, 

 well in the body wall as in the 

 wall of the tentacles, ambulacra, tube- c ciform b d y of Cucumaria crucifera 



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



feet, and ambulacra! papillae, there are sticopus japonicus ; 5, supporting plate 

 found enormous numbers of micro- from one of the tube-feet of stychopu* 

 scopically minute calcareous bodies of w nicus = 6 ' " sto i " of Hoiothum 



, . J . Murray i ; 7, rod from the ventral ambula- 



definite shapes (b Ig. 300). ihese give cial appendages of Oneirophanta mutabUis, 



the integument a firm and rough Th6el ; s > latticed hemisphere of Colochirus 



ooTisist-pnrv Thpir nrinriml sio-nifi cucumis - Sem P: 9. " wh eel" of Acantho- 



icy. principal signm- trochus mirabili8> 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. 



1 On the author's use of the term " perisomatie," see footnote, p. 362. 

 VOL. II 7 



