VIII ECHINODEBMATA— MORPHOLOGY OF SKELETON 337 



certain pieces of the calcareous ring of the Holothupioidea corre- 

 spond with the orals of other Echinodenns cannot at present be 

 determined. 



C. The Pepisomatie Skeleton, i 



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

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

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

 inatic skeleton must varj' inversely with that of the polar (apical and 

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

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

 and vice versCt. 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 perisomatic 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 Pelmaiozoa, 

 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. 



I. Holothupioidea. t^ »„« ,t. • , 



Fig. 300.— Microscopic calcareous bodies 

 T .1 . - ^ ii TT 1 11 * • 7 of Holothiiiioidea. 1, Aiichor and anchor 



In the cutis of the EoMlWlOtdea, pi^j, „f gynapta Inhaerens, O. P. M. ; 2, 



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



wall of the tentacles, ambulacra, tube- ""^if"™ tody of cucumaria crucifera, 



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



leet, and ambulacral papulae, there are sticopus japonicus ; s, supporting piate 

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

 scopically minute calcareous bodies of JJ''''"^"^ ^ ", "stool" of Hoiothu™ 



J J^ , , . . Murrayi ; i.rod from the ventral ambula- 



dennite shapes (Fig. 300). These give cral appendages of Onelrophanta mutabilis, 

 the integument a firm and rough ^heel ; 8, latticed hemisphere of Colochirus 



consistency. Their principal signifi- 

 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. 



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

 VOL. II Z 



cucumis, Semp; 9, "wheel" of Acantho- 

 trochus mirabilis, Dan. and Kor. 



