THE HYDROMEDUSAE 29 



in the family Plumularidae, where the modification of a hydro- 

 cladium results in the formation of a special organ termed the 

 phyla ctocarp, the complexity and completeness of which vary greatly 

 in different genera and species. (a) All the hydroids of a hydro- 

 cladium may be suppressed and replaced by gonophores (or blasto- 

 styles), which are guarded by the usual median and lateral nemato- 

 phores (Lytocarpus). (b) The hydroids, instead of being sessile in 

 the centre of the axis of the hydrocladium, project laterally out- 

 wards, their central position being occupied by the gonophores ; the 

 peduncles on which the hydroids project are produced each into 

 a long rib (homologous, according to Allman, with an elongated 

 median nematophore), which bears numerous lateral nematophores. 

 These ribs arch slightly over the gonophores (Acanthocladium). 



(c) All the hydroids of the hydrocladium, except one or two nearest to 

 the hydrocaulus, are suppressed ; the ribs of nematophores, formed 

 as described under (b), arch completely over the gonophores, forming 

 what is termed an open corbula (Aglaophenia attenuata, Fig. 37). 



(d) In a further stage, deducible from the last, the ribs join to 

 form a complete investment of the gonophores, except for one 

 (Aglaophenia filicula) or several (A. Macgillivrayi) apertures. In 

 this manner the simple hydrocladium becomes converted into a 

 closed corbula. 



ORIGIN OF THE GENERATIVE CELLS. The general description 

 of the origin and migration of the generative cells in Anthomedusae 

 applies also to this group, but the changes there mentioned as 

 affecting the structure and functions of the gonophore, and the 

 acceleration of the formation of the generative cells, are even more 

 marked in the Leptomedusae. In most cases the cells, although in 

 all probability migrants from the ectoderm, are first noticeable in 

 the endoderm ; only rarely are they confined throughout to the 

 ectoderm (male Campanularia). They may make their first 

 appearance on the manubrium and migrate on to the radial canals 

 of the medusoid, in which case they are probably always ecto- 

 dermal in position (Obelia) ; or they may appear in the endoderm 

 of the blastostyle (male Campanularia), of the hydrocaulus 

 (Gonothyraea), or of its lesser branches (female Campanularia). 

 When a free -swimming medusoid is not present, they migrate 

 from their place of formation into the gonophore, and generally 

 penetrate through the mesogloea into the ectoderm of the rudi- 

 mentary manubrium or of the sporosac, as in the Anthomedusae ; 

 they rarely ripen in the endoderm (Sertularella). For the whole 

 question see Weismann (10). 



ASEXUAL REPRODUCTION. Gemmation is of the same laminar 

 character in hydroids of the Leptomedusae as in those of the 

 Anthomedusae. In the gemmation of gonophores from a blasto- 

 style, it frequently happens that the ectoderm is multilaminar ; 



