Royal Physical Society. 203 



coats, in place of the gelatinous matter secreted in the cap- 

 sules of the male polyps (fig. 6). These ova, at first consisting 

 of a small transparent vitellus, containing a germinal vesicle 

 and germinal spot, become of an opaque yellow, white, or 

 crimson colour, by the granulation of the yoke, and rapidly 

 enlarge, until the female polyp is almost hidden under the 

 mass of immense ovaries with which it is burthened. 



26. The Sessile Generative Sacs of the polypary are deve- 

 loped from the tubes of the polypary itself. They resemble 

 exactly those of the reproductive polyp, and contain ova and 

 spermatozoa in the polyparies of different sexes. 



27. Three hinds of reproduction may be observed to obtain 

 amongst the hydroid zoophytes : — 1st, Poly pip arous, where the 

 young is discharged from the. ovarian capsule in the complete 

 form of a polyp, as in Tubularia ; 2d, Larviparous, where 

 the young are born as ciliated planarioid larvse, as in Clava, 

 or simple unciliated germ-masses, as in Coryne, afterwards 

 becoming polypoid ; and 3dly, Oviparous, where true ova are 

 discharged from the ovarian sacs, and the subsequent changes 

 into planarioid and polypoid forms take place after their leav- 

 ing the parent zoophyte, as in the subject of this paper. 



28. In Hydractinia, the ova, after extrusion from the 

 ovary, and their having undergone fecundation by the sper- 

 matic fluid of a male polypary, become segmented (as in fig. 

 7) by the usual process of yelk-cleavage, and are trans- 

 formed into transparent fleshy masses, in which may be detected 

 the rudiments of the endodermic and ectodermic tissues. 

 These masses presently become developed into taper cones 

 (fig. 8), which attach themselves by their bases, travel along 

 the surface of the glass in which they are kept, and congre- 

 gate on the side next to the light, like a forest of tiny masts. 

 They consist of an ectoderm, destitute of thread-cells, but 

 thickly studded with the soft spines I have before mentioned 

 (19), and tenaciously adhering to any body brought in con- 

 tact with them; and of an endoderm, — crimson, granulated, 

 and not vacuolated, inclosing a cavity which occupies the 

 axis of the cone along its whole length. Many of these larvse 

 have lived for several weeks with me without undergoing any 

 further change ; others have in a few days been developed 



vol. i. s 



