154 



HYDROIDEA. 



The development of Eucope polystyla (fig. 68), one of the 

 Campanularida^, deviates according to Kovvalevsky (No. 147) in 

 somewhat important points from the usual type. The whole 

 development takes place after the deposition of the ovum. The 

 segmentation results in the formation of a single-walled blasto- 

 sphere with a large central cavity (fig. 68 A). This cavity, 

 somewhat as in Ascetta, becomes filled up with a not clearly (?) 

 cellular material derived from the walls of the blastosphere, 

 which must be regarded as the hypoblast (fig. 68 B). The larva 

 elongates and becomes ciliated, and the epiblast at its two 

 extremities becomes thickened, and is stated by Kowalevsky 

 also to become divided into two layers. The alimentary cavity 

 appears as a slit in the middle of the hypoblast (fig. 68 C). The 

 cilia after a time disappear, and the larva then becomes fixed by 

 one extremity. It flattens itself out into a disc-like form, becomes 

 divided into four lobes, and covered by a cuticle (perisarc). 

 From the disc the stalk grows out which dilates at its free ex- 

 tremity into the calyx. 



In both the groups (Tubularia and Hydra) 

 having a ciliated planula stage, its absence may 

 be put down to an abbreviation of the develop- 

 ment, and in fact a two-layered quiescent stage, 

 through which the embryo passes, may be 

 regarded as representing the planula stage. 



The development of Tubularia, which has 

 been described in detail by Ciamician, takes 

 place in the gonophore 1 . The segmentation 

 is irregular and leads to the formation of an 

 epibolic gastrula, four large central cells con- 

 stituting the hypoblast 2 . The larva now elon- 

 gates, and grows out laterally into two pro- 

 cesses which constitute the first pair of 

 tentacles. At this stage it closely resembles 

 the larva; of some Medusae. Additional ten- 

 tacles are soon formed ; and a central cavity 

 appears in the hypoblast, the cells of which 

 have in the meantime become more numerous 

 (fig. 69). The tentacles are directed towards 



which are exceptional in not 



FIG. 69. LONGITUDINAL 

 SECTION THROUGH A LARVA OF 

 TUBULARIA MESEMBRYANTHE- 

 MUM WHILE STILL IN THE 

 GONOPHORE. The lower end 

 is the oral one. 



ep. epiblast; hy. hypoblast 

 of tentacle ; en. enteric cavity. 



1 Vide Ciamician, Zeit.f. win. Zool., Bd. xxxil. 1879. 



1 In examining the segmentation by means of sections I have failed to detect an 

 epibolic gastrula or such irregularity as is described by Ciamician. Prof. Kleinenberg 

 informs me that he has been equally unsuccessful. 



