XVI ALIMENTARY CANAL REPRODUCTION 459 



The only important variation from the type, as described in 

 Astcrias, is found amongst the Astropectinidae and Porcellanas- 

 teridae, where the anus is wanting. In Astropecten the rectum 

 and the rectal caeca still persist, but in Luidia even these have 

 disappeared. The rectal caeca are remarkably variable structures. 

 In Aster ias there are two, but in Pentacerotidae there are five 

 forked caeca, in Asterina five simple caeca, and in the Echin- 

 asteridae and Astropectinidae one large flat slightly 5-lobed 

 caecum. In the Asterinidae the pyloric caeca are remarkable 

 for the size of tlie enlarged basal portion in each radius, which 

 serves as a reservoir for the juices secreted by the branched 

 forks of the caecum. In Porcellanaster pacijicus the pyloric 

 caeca are vestigial, and in Hyphalaster moseri they are absent.^ 



The genital organs are, as we have seen, outgrowths from 

 radial branches of the genital rachis. In most species, as in 

 Asterias, they are limited to a single cluster of tubes on each 

 branch of the rachis, but in the Astropectinidae and Pentacero- 

 tidae each branch gives rise to a large number of clusters, 

 arranged in longitudinal series, each cluster having its inde- 

 pendent opening to the exterior. 



Asexual reproduction, as a regular occurrence, is not common 

 amongst Asteroidea. If, however, a Starfish loses some of its 

 arms, it has the power of regenerating the missing members. 

 Even a single arm will regenerate the whole Starfish. Now in 

 some cases (Astropectinidae, Linckiidae) Starfish will readily snap 

 off their arms on irritation. In TAnchia this occurs at regular 

 intervals and the separated arm forms a new individual. In one 

 of the Asterinidae, Asterina wega, a small Starfish with seven 

 arms, transverse fission regularly occurs, a portion with three 

 arms separating from one with four. The same is believed to 

 occur in two species of Asterias, and as has already been pointed 

 out, the repetition of the madreporite and stone-canal is, in many 

 cases, possibly connected with this tendency to transverse fission. 



Classification of Asteroidea. 



Whilst there is considenil)le agreement amongst the authorities 

 as to the number of families, or minor divisions of unequivocal 



^ Ludwig, "Scientific Results of the Expedition of the 'Alliatross' to the Tropical 

 Pacific"— "Asteroidea," 1905, pp. 91, 103. 



