DEVELOPMENT OF ST ABr FISHES. 



113 



Ing through the morula and gastrula stages, the cephalula 

 or larval stage is reached, the mouth, digestive sac and its 

 posterior opening being formed, a cephalic end being dis- 

 tinguished from a posterior end. Tho larva is now bilater- 

 ^ally symmetrical. At this time two lobes arise from each 

 |side of the mouth. These separate from their attachment 

 and form two distinct hollow cavities, and by the time the 

 larva attains the Brachiolaria stage the development of the 



Fig. 75. — Blpinnaria with fne star. 

 flsii Dudding from it. e, e', d', g, g\ 

 protuberances of tlie body comparable 

 with the " arms " of the Brachiolaria 

 figured in the adjoining engraving, 

 ft, mouth; o, vent of the larva; ^,germ 

 of th? Btar-flsh; h, ciliated digestive 

 tract; i, ambulacral rosette (germ of 

 the water-vessels). — After MtUlir, from 

 Gegenbaur. 



Fig. 76.— Brachiolaria 

 of Asteiias vulgaris, en- 

 larged, with the star-fish 

 ()•) developing at the 

 aboral end. e. median 

 anal arm; c", odd termi- 

 nal oral arm; /, brachio- 

 lar arm; /', branch of 

 water-tube (aiwO leading 

 into /" odd brachiolar 

 arm; /"', surf ace-warts 

 at base of odd brachiolar 

 arm/".— After A. Agas- 

 siz. 



body of the star-fish begins, for these two cavities subse- 

 quently develop into two water-tubes. On one of these cav- 

 ities the back of the star-fish is afterward developed, while 

 on the other the under side with the feet or tentacles arise. 

 The fully-grown larva is called a brachiolaria, as it was 

 originally described with this name under the impression 

 that it was an adult animal, as was the case with the plu' 



