CTENODISCUS. 113 



ing again the correspondence between the adult forms of the 

 lower orders and the phases of growth in the higher ones, 

 that these spines have an embryonic character. One would 

 naturally expect to find that these small spines of the adult Cri- 

 brella would differ from those of the other full-grown Star-fishes 

 chiefly in size, that they would be a somewhat modified pattern 

 of the same thing on a smaller scale ; but when examined under 

 the microscope, they resemble the spines of the higher orders in 

 their embryonic condition ; it is not, in fact, a difference in size 

 merely, but a difference in degree of development. The Cri- 

 brella moves usually with two of the arms turned backward, and 

 the three others advanced together, the two posterior ones being 

 sometimes brought so close to each other as to touch for their 

 whole length. 



Hippasteria. (Hippasteria phryyiana AG.) 



Beside these Star-fishes we have the pentagonal Hippasteria 

 (Hippasteria phrygiana AG.), like a red star with rounded points, 

 found chiefly in deep water, though it is occasionally thrown up 

 on the beaches. It has but two rows of large tentacles, termi- 

 nating in a powerful sucking disk. The pedicellariae on this 

 Star-fish resemble large two-pronged clasps, arranged principally 

 along the lower side. The pentagonal Star-fishes of our coast 

 are in striking contrast to the long-armed species we have just 

 described ; they are edged with rows of large smooth plates, and 

 do not possess the many prominent spines so characteristic of 

 the ordinary Star-fishes. 



CtenodiSCUS. (Ctenodiscus crispatus T>. & K.) 



The Ctenodiscus (Ctenodiscus crispatus D. & K., Fig. 147), an 

 inhabitant of more northern waters, but seeming also to be at 

 home here occasionally, is another pentagonal Star-fish. It lives 

 in deep water, and freqiients muddy bottoms. The peculiar 

 structure of their ambulacra has probably some reference to this 

 mode of living, for they are entirely wanting in the sucking disks 

 so characteristic of the other members of this class, and their 



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