CNIDARIA 105 



form of tlie sexually mature medusa until after a metamor- 

 phosis, which in most cases is rather complicated. 



In most of the Discophora hitherto studied development 

 takes place in the form of an alternation of generations 

 already described. This is wanting in the Lucernaridse 

 only, they representing a sexually mature scyphopolyp 

 stage, from the eggs of which individuals of the same form 

 arise. On the other hand, among the free-swiming acraspe- 

 dote medusae cases (Pelagia) of direct development are 

 known, in which a larva developed from the egg of the 

 medusa changes directly into the Ephyra stage. This is 

 looked upon as a case of coenogenetically abbreviated develop- 

 ment, since the formation of a non-sexually reproducing 

 nurse-form (Scyphistoma) is suppressed. 



Development of the Scyphistoma. — The develop- 

 ment of Aurelia (A. aurita and A. flavidula) is that of which 

 we have the most complete knowledge ; it has been made 

 known through numerous investigations — those of M. Saks 

 (No. 112), V. SiEBOLD (No. 114), L. Agassiz (No. 2), Glaus 

 (Nos. 102 and 103), Haeckel (No. 107), and Goette (No. 

 105). In the following we adhere chiefly to the description 

 of Goette, by whose inve.stigations a number of new points 

 of view have been gained. 



The eggs of Aurelia aurita pass from the ovary into the 

 gastral cavity of the parent, and from there through the 

 mouth into the folds of the oral arms, where, enveloped by a 

 slimy secretion fx-om the entoderm, they undergo embryonic 

 development as far as the stage of the swarming planula. 

 They are enveloped by a delicate vitelline membrane, which 

 is lost in the later stages of cleavage. 



By total and equal cleavage (Glaus) the egg divides into 

 a number of equal-sized blastomeres, which arrange them- 

 selves in a single layer about a comparatively small cleavage 

 cavity (coeloblastula). While, according to Glaus (in har- 

 mony with the statements of Kowalevsky), the gastrula-stage 

 is reached by means of a process of invagination,^ in which 



^ [The observations of Claus have been fully corroborated by the 

 recent investigations of Fbank Smith (No. VII., Appendix to Literature on 

 Scyphomeduste) on Aurelia jiaiidala. In this species the entoderm is pro- 



