THE HYDROZOA. 117 



nent, and the one or many solid mineral bodies which they 

 contain are inclosed in special envelopes. Their structure 

 appears to be more complicated in the Geryonidce than in 

 other Medusas. (Haeckel, loc. cit.) 



In some of those medusoid gonophores, the reproductive 

 elements are developed while the gonophore is still attached 

 to the hydrosoma, and then they always make their appear- 

 ance in the wall of the manubrium. But, in other cases, the 

 medusoid becomes detached before the development of the 

 reproductive elements, and, feeding itself, increases largely 

 in size before the ova or spermatozoa appear. Sooner or 

 later, however, the reproductive organs are developed, either 

 in the walls of the manubrial hydranth, or in those of the 

 canals of the nectocalyx of the medusoid. 



In an early stage of its existence, every hydrozoon is 

 represented by a single hydranth, but, in the great majority 

 of the Hydrozoa, new hydranths are developed from that 

 first formed, by a process of gemmation or of fission. In 

 the former case the bud is almost always an outgrowth or 

 diverticulum of the ectoderm and endoderm, into which a 

 prolongation of the cavity of the body extends. Sometimes 

 the hydranth formed by gemmation becomes detached from 

 the body ; but, in many cases, the buds developed from the 

 primary hydranth remain connected together by a common 

 stem or ecenosarc, and thus give rise to a compound body, or 

 hydrosoma. 



In many Hydrozoa, the ectoderm gives rise to a hard cu- 

 ticular coating, and in some of these ( Campanidaridaz, Ser- 

 tularidce, Fig. 13, 2), this cuticular investment, on the hy- 

 dranth, takes the shape of a case or " cell " — the hydrotheca 

 — into which the hydranth may be more or less completely 

 retracted. In other Hydrozoa, protective coverings are af- 

 forded to the hydranths by the development of processes of 

 the body-wall, which become thick, variously-shaped, glassy 

 lamellae. These appendages are termed hydrophyllia (Fig. 

 13, 3). 



Again, certain groups (the CalycophoridcB and most Phy- 

 sophoridaS) are provided with bell-shaped organs of propul- 

 sion, produced by the metamorphosis of lateral buds of the 

 hydrosoma. These nectocalyces have the structure of a med- 

 usoid, devoid of a manubrium. In others (Physophoridw), 

 one extremity of the hydrosoma is dilated, contains air in- 

 closed within a sac formed by an involution of the ectoderm, 

 and constitutes a float or pneumatophore ; while in yet others 



