CNIDARIA 113 



respects the phenomena of the embryonic development are essentially 

 the same as those we have described for Aurelia. By means of total and 

 equal cleavage, a coeloblastula arises, out of which by infolding an 

 invaginate gastrula develops, whose i^rostoma remains open for a con- 

 siderable time, but finally closes completely. From observations by 

 BuscH, it appears as if reproduction of the embryo, by means of longi- 

 tudinal division, frequently took jDlace at this stage. This recalls the 

 occurrence of fission in the blastula of Oceania armata, according to 

 Metschnikoff (p. 49). In the stage of the ciliated planula the larvffi 

 of Chrysaora jiass from the ovary into the gastral cavity of the i^arent, 

 and thence to the outside world through its mouth. A glandular modifi- 

 cation of the ectoderm of the anterior pole of the larva, by means of which 

 the attachment subsequently takes place, can be recognized, whereas the 

 posterior (oral) pole is characterized by the ajipearance of nettle capsules 

 (Claus). 



The opaque whitish or yellowish eggs of Nausithoe are laid singly, and 

 are characterized by a gelatinous envelope, provided with nettle capsules 

 (0. Hertwig). Cleavage is here total, and in the first stages unequal, 

 though finally a coeloblastula with walls of nearly uniform condition is 

 produced by the gradual obliteration of the great differences in size be- 

 tween the blastomeres. The blastula changes into an oval, ciliated swarm- 

 ing larva, the cells of which are thickened at the posterior pole, where 

 the gastrula invagination takes place. After invagination the blastopore 

 becomes completely closed. Metschnikoff (No. 12), to whom we owe 

 the knowledge of these processes, was able to observe the attachment of 

 the planula, which is accompanied by the development of a discoid 

 basal expansion, and its metamorjjhosis into a small scyphojDolyp pro- 

 vided with four tentacles and covered with a thin layer of ijeriderm, so 

 that metagenesis has been proved for this form also. Metschnikoff 

 believed that he was justified in assuming that the Spongicola fistularis 

 of F. E. Scholze (Stephanoscyphus mirabilis, Allman), which is para- 

 sitic in sponges, and in which Kowalevsky seems to have observed a kind 

 of strobilization, is the Scyphistoma form of Nausithoe. 



Strobilization. — -The simplest form of reproduction of 

 young medusas is represented by the monodiac strohila (Fig. 

 59 A), occasionally observed even in Aurelia. In this case 

 only one young medusa (Ep/i?/»"«) separates from the Scyphis- 

 toma. While the adoi-al tentacle-bearing portion of the scypho- 

 polyp is by gradual changes converted into the form of the 

 Ephyra, it becomes separated by means of a circular, trans- 

 verse furrow from the basal portion of the body, and finally 

 detaches itself completely. The basal remnant can, by re- 

 generation of the oral portion, grow again into a complete 



K. H. E. T 



