viii ECHINODERMATA MORPHOLOGY OF SKELETON 337 



certain pieces of the calcareous ring of the Holothurioidea corre- 

 spond with the orals of other Echmoderms 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. Where the polar systems form only a small part of 

 the body wall the perisomatie 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, 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 Ophiuroidea, 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. FIG. 300.-Microscopic calcareous bodies 



T ,i , . ,. , , TT 7 ,7 . . 7 of Holothurioidea. 1, Anchor and anchor 



In the cutis of the Holotlmnoidw, plate of Synapta inll8erens> 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- cruciform body of Cucumaria crucifera 



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



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

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



sormiVallv mirmrp raloarprms bodies of J a P nicus > 6 > "stool" of Holothuria 



scopicany i es^ o.[ Murrayi . 7> rod from the ventral ambula . 



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



the integument a firm and rough Theel ; 8 > latticed hemisphere of coiochirus 



. , m , . i . .r. cucumis, Semp ; 9, " wheel " of Acantho- 



consistency. Their principal sigmn- trochus mirabiliS) 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 Z 



