228 
rOrULAR SCIE^’CE REVIEW. 
But the diffusion of the species is secured by a true sexual 
reproduction ; and a peculiar interest attaches to this portion of 
the life-history of the zoophyte. Here again we trace a striking 
analogy to the phenomena of plant-life. At certain seasons, 
besides the buds that are developed into polypites, others make 
their appearance on the zoophyte, which are destined to origi- 
nate and mature the seed of new colonies, and correspond with 
the flower-bud of the plant. The alimentary and reproductive 
functions are distributed amongst two classes of zooids ; one 
provides for the existing commonwealth, the other lays the 
foundations of new communities. In the tribe now under 
consideration, these reproductive buds are alwa}^s inclosed in 
horny capsules, which are distributed over the branches 
amongst the calycles, and often exhibit the most graceful urn- 
like shapes. In some cases the reproductive zooids continue 
permanently attached like the polypites, and the embr}ms, when 
mature, escape from them into the water. They may be de- 
scribed as closed sacs, into which the general cavity of the body 
extends, so that they are freely visited by the nutrient currents ; 
and between the two layers that compose the walls of the sac, 
as of every other portion of the Hydroid organism, the genera- 
tive elements are produced. These fixed reproductive sacs are 
structurally identical with the polypite up to a certain point ; 
but the portions of structure that are essential to its half-inde- 
pendent existence, the mouth and tentacles, are suppressed in 
the sexual zogid. It receives its nutriment from the general 
circulation, and devotes its energies, not to the capture of food, 
but to the elaboration of the reproductive elements. Several of 
these buds, borne on an offshoot from the coenosarc, are usually 
met with in the capsule, which protects them through the course 
of their development, and allows free egress to the embryo, when 
mature. (Plate XLVI. fig. 5oo). 
The sexual members of the Hydroid colony are not always, 
however, of so humble a structure, nor do they always maintain 
their connection with the community. In many genera they 
take on a highly complex organisation, and lose the sedentary 
habits ot their tribe ; the}" are furnished with the means of 
locomotion, and simple organs of sense, and at a certain point 
of their development detach themselves from the parent stock, 
and lead a free and active existence, during which they fulfil 
their functions and then perish. These free sexual zooids 
exhibit a form of structure, no less than a mode of life, which 
contra.sts strikingly with tliat of the plant-like zoophyte and its 
ordinary polypites. (Plate XLV. fig. o.) They consist of a 
somewliat cylimlrical and sac-like body (Plate XLV. fig. 5a), 
suspended in the centre of a filmy transparent bell, which acts 
as a float, and bears it uf), while, by its contractile movements, 
