162 SYNOPTIC COLLECTION. 



Since specialized Asteroidea occur with the Cystoidea 

 in the Cambrian formations, it is impossible for the 

 former to have descended from the latter. We must, 

 therefore, look for the ancestors of both groups in the 

 pre-Cambrian rocks, and it seems most likely that such 

 a form will combine the characters of both Cystoidea and 

 Asteroidea. Most of the Palaeozoic starfishes, like those 

 of to-day, were free -moving and crawled with the oral or 

 actinal side downward. If we suppose an ancient star- 

 fish to be attached by a stem extending downward from 

 the middle of the aboral or abactinal surface, we have 

 a striking resemblance to a stalked Crinoid. In such a 

 case the mouth is in the middle of the upper side and 

 the ambulacra run out from it into the arms. When, 

 however, the starfish became free-moving, it turned upon 

 the ventral side and the tentacles which had been use- 

 ful in catching food became modified in time into loco- 

 motor organs. 



It seems probable that the ancestral starfishes had a 

 pentagonal body with spines slightly developed, and two 

 rows of plates in each ambulacrum, the plates of one 

 row alternating with those of the other. One of the 

 descendants of such a form may be the living Ctenodiscus 

 (No. 291, C. crispalus D. Kor.). When young (No. 

 291 a), it has a dome-like body which becomes flatter with 

 age (No. 291 b, c). The adult has a large disc and short 

 arms, giving it a pentagonal outline. The plates of the 

 aboral side (No. 291 b) are small and leathery, the spines 

 being slightly developed. Near the center is the anal 

 tubercle. Each ambulacrum on the ventral side (No. 

 291 c) consists of two rows of alternating plates which 

 are broad and not crowded closely together. There are 

 two rows of holes for the tentacles which are without 

 sucking discs. Every ambulacrum is flanked on each 

 side by a row of good-sized interambulacral plates, and 

 outside of these are well developed marginal plates (No. 

 291 c). 



