248 THE STELLEROIDEA 



madreporites, but in the Solasteridae there is but one ; on the 

 other hand, many five-rayed starfish, such as Asterias capensis, have 

 more than one. The stone-canal is repeated, especially in forms 

 which reproduce asexually. 



There is no anus in the Astropectinidae, the members of which 

 are more primitive than Asterias in many respects. The radial caeca 

 of the stomach remain constant, but the rectal caeca vary in number 

 and arrangement ; they are increased to five in many genera, and 

 in Culcita each of the five caeca branches into two. 



The "papulae" or branchial vesicles, which, in Asterias, rise 

 from all parts of the external surface, are limited among Phaner- 

 ozonia to the abactinal surface, and to the area enclosed by the 

 supra-marginal plates. 



The Genital Organs are, as a rule, less concentrated than in 

 Asterias. The glands are repeated along the arms ; in the simplest 

 cases each series discharges its products by a single sieve-plate. 

 The extreme case is in Brisinga, where there are a series of separate 

 glands along the arm, one pair to each segment, and each gland 

 discharges by a separate aperture. 



In Asterina, giblosa the genital orifice is on the ventral or 

 actinal surface, as in Ophiuroids. 



The development of the Asteroids is generally indirect, the larva 

 passing through a metamorphosis. The typical form of larva is 

 the Bipinnaria, which passes through an Auricularia stage and thus 

 resembles the larvae of the Holothurians rather than of the 

 Echinoidea or Ophiuroidea (see p. 5). In some cases the Bipinnaria 

 develops into a Brachiolaria by the division of the frontal process 

 of the larva into three branches. In some genera, such as Asterina 

 (the development of which has been studied with especial care), 

 the Bipinnaria stage is never developed, although the larvae are 

 free-swimming and undergo metamorphosis. In other cases, e.g. 

 in Blakiaster and Pteraster, the development is direct ; in the 

 former case, the ova develop in "arcade-like spaces" between the 

 paxillae of the abactinal surface ; in the latter there is a large 

 marsupium formed by the presence of a supra-dorsal membrane 

 rising above the abactinal surface. 



Asexual reproduction is not uncommon ; it results either from 

 a division of the body, approximately into halves, or by regrowth 

 of the disc from an arm that has been thrown off. The latter 

 method occurs especially in Linckia, and with the first appearance 

 of the disc the starfish is said to assume the comet form. 



Proceeding to the Systematic Description of the orders and families, 

 we may sum up the foregoing characters in the following Diagnosis of 

 the Sub-Class. 1 The Asteroidea are eleutherozoic, actinogonidial, and 



1 Emended from Bell (4), p. 19 ; the terms are explained, antea, p. 2-37. 



