96 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



southern coasts of Australia we find Comatulella brachiolata (part 1, fig. 77, 

 p. 130), a curious species with very short and very stout arms and very short 

 cirri composed of very short segments, which is evidently a cold-induced form 

 derived from the ( 'otnatula type (figs. 184, 185, pp. 100, 102). In Comanthus wahl- 

 bergii of South Africa and in (.'. trichoptcra and its allies of southern Australia, 

 New Zealand, and Tasmania the same feature is evident, though in the C. trichop- 

 tera group a distinctive type of cirrus structure prevents these organs from being 

 strictly comparable with those of the other species of the genus. 



Although an unusually warm habitat ordinarily induces the lengthening of 



the comatulid arm, under certain con- 

 ditions and upon certain arms it has 

 exactly the opposite effect, causing 

 these arms to become short and stout. 

 The effect of a cold habitat appears to 

 be to render slower and more difficult 

 the formation of the skeleton, retard- 

 ing the deposition of lime to a much 

 greater degree than it inhibits the 

 other body processes, possibly as a re- 

 sult of the increased solubility of cal- 

 cium carbonate (CaCO 3 ) at low tem- 

 peratures. Hence, in cold water forms 

 during the growth of the arms the 

 various organs attain their definitive 

 form when the brachials are still rela- 

 tively short, and the brachial skeleton 

 never attains the weight and density 

 of the brachial skeletons of warm- 

 water species. The shortening of the 

 brachials and therefore of the arms 

 also as a result of the action of cold 

 always affects all the arms to the same 

 degree. 



The species of Comasteridse in 

 which the posterior arms are short 

 and ungrooved all occur in the littoral 

 zone of the Tropics, where the temperature of the water is high. Species from deep 

 or relatively cold water always have all of the arms of the same length and similar, 

 as in the endocyclic forms. There seems to be a definite connection, therefore, be- 

 tween a high temperature and modification of the posterior arms. 



Ungrooved arms are especially remarkable for the great development upon 

 them of the sexual organs, which are more numerous and larger than those upon 

 the other arms. 



182 



Fio. 182. CENTRAL PORTION AND ONE POST-RADIAL 



SERIES OF SPECIMEN OB 1 NEMASTEtt RUBIG1NOSA COL- 

 LECTED BY THE " CHALLENGER," AT BAHIA, BRAZIL. 



