THE STELLEROIDEA 259 



sharply marked off from the disc. Abactiual skeleton absent, or present 

 only on the ovarial regions at the bases of the arms. No intermediate 

 actinal plates or interbrachial septa. Genera Brisinya, Asbjornsen ; (?) 

 Brisinyaster, Lor. ; Colpaster, Slad. ; Freyella, Slad. ; (?) Gymnobrisinga, Stud.; 

 (?) Hymenodiscus, Per. ; Labidiaster, Liitken ; Medusaster, Stiirtz ; Odinia, 

 Per. This remarkable family was originally founded for the genus Brisinga, 

 which has many primitive characters. The arms are small and sharply 

 marked otf from the disc. There are no ampullae connected to the podia; 

 the generative organs consist of a series of small isolated glands along the 

 arms. The genus was accordingly at first regarded as very primitive in 

 structure and affording in some ways a link with the Ophiuroids. Later 

 authors, however, such as Ludwig and Sladen, entirely repel this view 

 and regard the Brisingidae as allied to the Asteriidae, and extremely 

 specialised rather than primitive. Sladen concludes, " In my opinion 

 the Brisingidae are true cryptozonate Asterids, very nearly related to the 

 Asteriidae, Pedicellasteridae, Heliasteridae, and Echinasteridae, and prob- 

 ably derived from a common ancestor, the divergence of form and the 

 peculiarities of structure now exhibited by Brisinga being the result of 

 modification produced by the extreme isolation and the exigencies of 

 the abyssal depths in which the family has existed" (48, p. 593). But 

 in Colpaster and Freyella the genital glands are limited to swellings 

 at the base of the arms ; and although the arms are sharply marked 

 off from the disc, at least six-sevenths of the arm has no extensions of 

 either alimentary canal or generative organs. The arm-ossicles of Freyella 

 tuberculata are identical in character with those of Ophiurina, and they 

 differ from Ophioyeron only by the absence of adambulacral ossicles. 

 Sladen gives no diagnosis of the class Asteroidea, so that it is not 

 quite clear on what characters he would base the separation of Asteroids 

 and Ophiuroids. But no known diagnosis of the Asteroidea would 

 include Colpaster and Freyella, and exclude forms universally admitted to 

 be Ophiuroids. If the more primitive types of Brisingidae are a recent 

 degenerate offshoot from the Asteriidae or some allied family, then a type 

 of structure, practically indistinguishable from that of the Ophiuroidea, 

 has been twice independently evolved. It seems therefore that Perrier is 

 probably correct when he regards the Brisingidae as the most primitive 

 instead of as the most special of living Asteroids. 



The following fossil Asteroids are not sufficiently known for determina- 

 tion : 



Calliaster, Trautschold ; Goelaster, Sandb. non Agass. ; Cribrellites, Tate ; 

 Cupulaster, Fritsch ; Plumaster, Wright ; Trichotaster, Wright. 



SUB-CLASS 2. OPHIUROIDEA. 



The Ophiuroidea form a sub-class of the Stelleroidea, including 

 the sand-stars, brittle stars, and branching stars. The typical 

 members of the class differ from the typical Asteroids by having 

 the arms sharply marked off from the disc as appendages, and by 

 not having a groove along the ventral side of the arms. These 



