THE HYDROMEDUSAE 29 



in the f.iiiiily Plumularid.ie, where the modification of a hydro- 

 cladium results in the formation of a special organ termed the 

 phyladorarp, the complexity and completeness of which vary greatly 

 in ditlerent genera and sjjccies. {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- 

 j)hores (Lytocarpus). (h) The hydioids, instead of being sessile in 

 the centre of the axis of the hj'drocladium, project laterally out- 

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

 peduncles on which the hydroids jtroject 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 (h), arch completely ovei' the gonophores, foiming 

 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 convertefl into a 

 closed coibula. 



Okioix of the Genkrativ?: Cells. — The general descrifjtion 

 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 Le])tomedusae. In most cases the cells, although in 

 all probability migrants from the ectoderm, are first noticealjle in 

 the endoderm ; ordy 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 (Obclia); or they may appear in the endoderm 

 of the blastostyle (male Campanularia), of the hydrocaulus 

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

 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 lipen in the endoderm (Sertularclla). For the whole 

 fjuestion see Weisniann (10). 



Asexual Kei'HODUCTION. — Cemmafion is of the same laminar 

 character in hydroids of the Leptomedusae as in those of the 

 Anthomedusae. In the gemmation of gonophores from a l>lasto- 

 style, it frequently happens that the ectoderm is multilaminar ; 



