THE DEVELOPMENT OF THE CORALLIGENA, 141 



veloped from its oral end. It can hardly be doubted that the 

 intermesenterio chambers are diverticula of the primitive en- 

 terocoele ; but the exact mode of their origin needs further 

 elucidation. 



Lacaze-Duthiers ^ has recently thrown a new light upon 

 the development of the Coralligena^ and particularly of the 

 Acthiim {Actinia, Sagartia, Bimodes). These animals are 

 generally hermaphrodite, testes and ovaria being usually found 

 in the same animal, and even in the same mesenteries ; but 

 it may happen that the organs of one or the other sex are, at 

 any given time, exclusively developed. The ova undergo the 

 early stages of their development within the body of the 

 parent. The process of yelk division was not observed, and 

 in the earliest condition described the embryo was an oval 

 planula-like body, composed of an inner colored substance 

 and an outer colorless layer. The outer layer (epiblast = ec- 

 toderm) soon becomes ciliated. An oval depression appears 

 at one end, and becomes the mouth ^ and gastric sac, while, at 

 the opposite extremity, the cilia elongate into a tuft. The 

 ectoderm extends into and lines the gastric sac, while the in- 

 terior of the colored hypoblast becomes excavated by a cav- 

 ity, the enterocoele, which communicates with the gastric sac. 

 In this condition the embryo swims about with its oral pole 

 directed backward. 



The oral aperture changes its form and becomes elongated 

 in one direction, which may be termed the oral axis. The 

 mesenteries are paired processes of the transparent outer 

 layer (probably of that part which constitutes the mesoderm) 

 which mark off corresponding segments of the enterocoele. 

 The first which make their appearance are directed nearly at 

 right angles to the oral axis near, but not exactly in, the 

 centre of its length. Hence they divide the enterocoele into 

 two primitive chambers, a smaller (A) at one end of the oral 

 axis, and a larger (A') at the other. This condition may be 

 represented by A-4-A'; the dots indicating the position of 

 the primitive mesenteries, and the hyphen that of the oral 

 axis. It is interesting to remark that, in this state, the em- 



1 " Developpement, des CoralUaircs." {ArcMves de Zoologie experim^ntale^ 

 1872.)^ 



2 Kowalewsky describes the formation of a ffastrula by invagination in a spe- 

 cies of Actinia and in Cereanthus^ the aperture of invacfination becoming the 

 mouth (llofmann and Schwalbe, " Jahresbericht," Bd. II., p. 269). In other 

 species of Actinia and in Alcijonium^ the planuhx seems to delamiuate. Ordi- 

 nary yelk division occurs in some Anthozoa^ while in others {Alcyonium) the 

 process rather resembles that which occurs in most Arthropods. 



