DEVELOPMENT. 



141 



half proceeding from one calcification centre ; in echinoplutei 

 the skeleton arises from five or six centres. 



The bipinnaria larva, found only in the Asteroidea, has two 

 ciliated rings, one preoral and the other longitudinal and postoral 

 (p. 137 and Fig. 89 B). In possessing a well-marked preoral 

 lobe it resembles the auricularia, and its two ciliated rings must 

 be regarded as having been derived by the division of the 

 single band of the auricularia, a view which is supported by the 

 fact that in some species (e.g. Asterias rubens, A, glacialis, 

 Astropecten) they are at first connected dorsally. Further, 

 Driesch* has shown that in some artificially reared bipinnariae 

 there is only one band. 



As in the pluteus a series of arms is 

 formed along the lines of the two ciliated 

 bands, and sometimes three arms, not 

 connected with the bands and covered 

 with warts, are formed in front of the 

 preoral arms at the anterior end of the 

 frontal area ; these are the brachiolar 

 arms and the larva bearing them the 

 Brachiolaria larva, f The arms are with- 

 out calcareous rods. 



12 



The median brachiolar arm replaces the ven- 

 tral median arm and bears at its base a sucker J 

 by wliich the larva attaches itself during the 

 metamorphosis. The ciliated band appears, as 

 a rule, not to pass on to these arms (except in 

 Bipinnaria papillata). 



FlG. 97. Bipinnaria elegans. 

 frontal area ; 2 preoral arm ; 

 3 anterior, 4 posterior trans- 

 verse portion of the ciliated 

 band ; 5 postoral, 6 postero- 

 lateral, 7 postero-dorsal arm ; 

 8 anal area ; 9 oral depression ; 

 10 antero-dorsal, 11 ventro- 

 median, 12 dorso-median arm 

 (after Mortensen). 



In the bipinnaria (and brachiolaria) the frontal area is well 

 developed and surrounded by the preoral ring of cilia. There 

 is a median ventral anterior arm and a median dorsal anterior 

 (Fig. 97), neither of which is present in the auricularia or in the 



* Arch. f. Entwickelungsmechanik, 20, 1906, p. 13. 



f It is possible that the brachiolar arms, which serve for temporary 

 attachment, are present in a later stage of all bipinnariae, for in the only 

 two cases in which the life-history is fully known (Asterias glacialis, A. 

 vulgaris) they are formed. 



J It is probable that in all bipinnariae this disc, which serves for 

 fixation during the metamorphosis, is present at a later stage in all cases, 

 for not only is it present in the two cases mentioned in the last note, but 

 it occurs in all cases thus far examined where a shortened development 

 due to food yolk has brought these stages within range of easy observation 

 (Ludwig, op. cit. p. 134). (Cf. Anasterias, Asterias antarctica, Asterina 

 gibbosa, Cribrella sanguinolenta.) 



