APPENDAGES OF ECHINODEKMA. 



199 



functions as ventral, are retained in Psoitis, while the other two 

 which belong to the portion of the surface of the body, which func- 

 tions as the dorsal, are atrophied. In Cuvieria this modification of 

 parts is extended so as to result in the disappearance of the three 

 ventral ambulacra. 



A complete degeneration of the ambulacra takes place in the 

 Synaptce, and the radiate organisation, which is implied by the 

 division into ambulacra, is thus lost; they have followed out the 

 line of modification indicated in those Asteroida, where the rays 

 begin to lose the organs belonging to them, as if in preparation for 

 a centralised organisation. 



Appendages. 



§ 



162. 



The organs appended to the integument, which may be regarded 

 as appendages, are not so various as in the Vermes. Of such parts 

 the sucking or ambulacral feet must be placed foremost, for 

 they form the most common arrangement, and belong to the typical 

 Echinoderm organisation, and are evidently derived from an ancestor 

 common to the whole group. They are tubular, and generally 

 cylindrical processes of the body-wall, which not only agree in being 

 arranged in rows (in accordance with the metamerism of the rays), 

 but also in their most essential points of structure, with the parapodia 

 of the Annelides : on the whole they are more simple in character. 



Fig. 100. Diagram of the cross section of an arm, A, of Asteracanthion rubens; 

 B, of Ophiura texturata. p Ambulacral feet, p' Ampulla?. t Dermal tentacles. 

 n Nerve-chords, w Ambulacral plates, in Muscles, a Ambulacral vein, b Ventral 

 plate, c Lateral plates, d Dorsal plate, h Calcified portion of the integument 



(after Wilh. Lange). 



(Fig. 100). As they have always much the same structure they 

 vary but little in function. 



The free end of these tubular structures (p) is either flattened 

 out, or provided with a sucker-like termination (Echinoi'da) ; or it 

 has a conical tip, or is rounded off (many Asteroida) ; sometimes it is 

 provided with a rounded head. Others are provided with lateral 

 indentations or secondary processes (Ophiurida and Crmoida) ; these 



