THE DEVELOPMENT OF THE ASTERIDEA. 



481 



has pointed out that, in Brisinga endecac?iemos, the genitalia 

 are numerous distinct glands, arranged in two series, one on 

 each side of the middle line of the central half of each ray. 

 Each of these ovaries or testes has a separate aperture. 



In seme Star-fishes, as in some Holothurids, the embryo 

 passes into the Star-fish form without any free larval stage. 

 But, more usually, an Echinopaedium is formed in the same 

 way as in the Holothurians, though it presents differences 

 in the arrangement of its ciliated bands, and especially in 

 their prolongation into numerous lobes or narrow processes, 

 as in the remarkable form originally named Bipinnaria. 

 (Fig. 135, D D', and Fig. 138). It has no calcareous skel- 

 eton. 



fe 



Fig. 138.— A youtis: Asterid larva (Bipinnaria. after Miiller). — A, ventral, B, lateral, 

 views of larva (Bipinnarin). C. Bipinnaria with rudiment of the Star-fish : a, 

 mouth : b, oesophagus ; c. stomach ; </, intestine ; o, anus; x, ventral. y % dorsal, 

 side of the anterior end of the body ; d, d / , ciliated hands ; h, caecal diverticulum 

 forming 1 the rudiment of the amhulacral vascular system, and opening externally 

 by the pore g. 



According to the observations of Prof. A. Agassiz, 1 which 

 have been confirmed by Metschnikoff and Greef, the ambu- 

 lacral vessels commence as diverticula of the stomach, which, 

 becoming detached from the alimentary canal, give rise to the 

 peritoneal cavity, and to all the substance of the body be- 

 tween the endoderm and the ectoderm. 2 A portion of one of 

 these diverticula, however, separates itself from the rest, 



1 " Embryology of the Star-fish." (" Contributions to the Natural History 

 of the United States," v., 1864.) The species, the development of which is 

 described in this important memoir, are Aster acantliion pallid us and A. bery- 

 linvs. 



2 Probably independently-developed mesoblastic cells contribute to the 

 formation of the mesoderm, as in the Holothurids. 



