METAZOA ECHINODERMA. 193 



extending from one end to the other. The mouth and 

 anus are both terminal. 



Apoda. The reduction of parts is going on in the 

 group to which Caudina arenata Stimp. (No. 366), 

 belongs. The young and adult, both seen in No. 366, 

 are similar in external appearance. The rows of feet 

 have disappeared, and the water-vascular system is, there- 

 fore, much reduced. Ludwig has shown that an allied 

 form, Chirodota rotifera Pourtales, has a stage in which 

 it loses its madreporic body and the stone canal detaches 

 itself from the dorsal wall, becoming enclosed within the 

 perisoma. 



The most specialized of all the Holothuria is Synapta 

 (No. 367, S. glabra Semper), found in the shallower 

 waters of the shore region. 



The body has become modified and is extremely elon- 

 gated. It contains spicules in the shape of anchors and 

 plates. The feet have disappeared and the radial ambu- 

 lacral vessels are also wanting, so that the water-vascular 

 system is reduced to a ring around the mouth. There is 

 no respiratory tree and altogether the genus is a good 

 illustration of specialization by reduction. 



To recapitulate briefly : The pre-Cambrian ancestor of 

 - the Echinoderms was probably free swimming and may 

 be represented by certain larvae of existing Echinoderms. 

 The Palaeozoic ancestors, however, were with little doubt 

 attached forms. Of these the Cystoids and Blastoids 

 had a more or less globular body which was either sessile 

 or fastened by a stem. The body was covered with 

 plates which at first were placed together irregularly but 

 in later forms were arranged in regular circles. The 

 oral surface was above and the aboral below. By the 

 differentiation of areas of plates, called ambulacra, and of 

 feathery pinnules, the apparatus for catching food and 

 carrying it to the mouth became more efficient. Pores 

 through the body wall admitted water to the respiratory 

 organs or hydrospires. 



