CO2LENTERATA. 127 



in Ascetta, becomes filled up with a not clearly (?) cellular material 

 derived from the walls of the blastosphere, which mast 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 

 extremity into the calyx. 



In both the groups (Tubularia and Hydra) which are exceptional in not 

 having a ciliated planula stage, its absence may be put down to an 

 abbreviation of the development, 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 constituting 

 the hypoblast 2 . The larva now elongates, and 

 grows out laterally into two processes which 

 constitute the first pair of tentacles. At 

 this stage it closely resembles the larvae of 

 some Medusae. Additional tentacles are soon 



formed ; and a central cavity appears in the e n \, 



hypoblast, the cells of which have in the mean 

 time become more numerous (fig. 69). The 

 tentacles are directed towards the aboral side, 

 which is considerably more prominent than 

 the oral one They contain a hjpobtato % 

 axis. The aboral end continues to grow and TDBULAEIA HESEMBBYANTHE- 

 the tentacles gradually assume a horizontal MUM WHILE STILL IN THE GOKO- 

 position. A constriction now appears, dividing PHORE. The lower end is the 

 the larva into an aboral portion which will one> 

 eventually form the stalk, and an oral portion. , P- epiblast ; hy. hypoblast 

 . . J , , ' L , , of tentacle ; en. enteric cavity. 



At the apex of the latter a row ot short 



tentacles the future oral tentacles now appears. The larva has at this 

 stage the form known as Actinula. In this condition it becomes hatched, 

 and shortly afterwards it becomes fixed by the aboral end and grows into 

 a colony. 



The development of Myriothela (Allman, No. 150) takes place on the 

 Tubularian type. The ovum invested by a delicate capsule becomes freed 

 by the rupture of the gonophore, and is then taken up by the remarkable 

 claspers characteristic of the genus. In the claspers it becomes fecundated 

 and undergoes its further development. After segmentation a gastric 



1 Vide Ciamician, Zeit. f. wiss. Zool., Bd. xxxn. 1879. 



2 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. 



