22 THE HYDROMEDUSAE 



SEXUAL REPRODUCTION. As a rule the female generative cells 

 (ova) and the male cells (spermatozoa) are formed in different 

 colonies ; they sometimes occur on different individuals of the same 

 colony (Dicoryne), or on the same blastostyle (Myriothela) ; they 

 rarely occur in the same individual (Hydra). 



In some cases there is but one ovum in each gonophore 

 (Eudendrium) ; more commonly, one cell at a time, out of many 

 " potential ova," is fertilised and develops, the remainder serving 

 as its food (Tubularia). The spermatozoa are always extremely 

 numerous. They escape by rupture of the tissues of the parent, 

 and swim freely in the water. In most medusoids the ova are 

 discharged in the same manner ; in most sporosacs and sessile 

 gonophores the ova are fertilised by spermatozoa, which penetrate 

 to them through the tissues. Segmentation of the ovum generally 

 produces a blastula (Fig. 1), a larva consisting of a single layer of 

 cells arranged round a central cavity, the blastocoele. By karyo- 

 kinetic cell division fresh cells are budded from the outer 

 layer into the blastocoele, which they ultimately obliterate. This 

 process is the formation of a diblastula (gastrula) by delamination 

 (Fig. 2) ; the outer cells are the future ectoderm, the inner mass 

 will give rise to the endoderm. The ectoderm becomes ciliated, the 

 diblastula elongates into the larval form termed a planula; at this 

 stage it generally leaves the parent and swims freely in search of 

 an appropriate site. To this it affixes itself, and sends out rooting 

 processes (hydrorhiza). A coelenteron becomes excavated in its 

 interior; and the appearance of mouth and tentacles, arid the 

 differentiation of cell forms, convert it into a hydroid. In some 

 cases the larva is not freed from the parent till this stage (actinula- 

 larva of Tubularia). In Hydra the ectoderm of the diblastula 

 secretes horny protective coatings, in which it passes a long 

 resting stage at the bottom of a pond ; a ciliated planula stage 

 does not occur in its history. 



In some cases the planula, instead of developing mouth and 

 tentacles, grows after fixation into a branching hydrorhiza, and 

 gives origin to hydroids by gemmation (Mitrocoma Metschnikoff, 

 13). 



ORDER 2. Leptomedusae (Calyptoblastea). 



DEFINITION. Hydromedusae with a regular alternation (meta- 

 genesis) of a sterile hydroid generation with a sexual generation 

 of medusoids or other gonophores. The perisarc typically forms 

 hydrothecae into which the hydroids are completely retractile, and 

 rigid permanent gonothecae which completely envelop the blasto- 

 styles and gonophores. The chief sense organs of the medusoids 

 are ocelli and otocysts ; the otoliths are the products of ectoderm 

 cells. The generative organs lie on the radial canals. 



