62 MORPHOLOGY. 



As a general rule, there is no perceptible difference between the male and female colonies of the 

 same species of hydroid, either in the trophosomc or the gonosome, beyond what is, of course, pre- 

 sented by the generative elements themselves. In some cases, however, the difference is sufficiently 

 well marked. Thus in Sertularia famaiisca the male and female gonangia (woodcut, fig. 26) differ 

 strikingly from one another ; for the male gonangia are compressed, somewhat obcordate recep- 

 tacles, with a short terminal tubular aperture ; while the female are oval for about the proximal 

 half of their height, and then become trihedral with the sides diverging upwards, the whole being 

 terminated by a three-sided pyramid whose edges are cut into two or three short teeth, and the 

 basal angles prolonged into a short spine.' 



So also in Serf/ikiria romcca a well-marked difference may be seen. The male gonangia 

 are here of a conical form, curved near the apex, which is their point of attachment, and provided 

 with six longitudinal ridges in the form of thin projecting lamellas, each of which terminates at 

 the distal extremity in a free-pointed process which arches over the summit of the gonangium. 

 In the female gonangium (woodcut, fig. 23) the longitudinal ridges are eight in number, while 

 two opposite ones being greatly more developed than the others give to the gonangium the very 

 elegant and striking form which caused Ellis to compare it to a " lily or pomegranate-flower just 

 opening." A very similar difference exists between the male and female gonangia of Sertularia 

 falla.r, and generally in the group which under the name of Biphasia, Agassiz has separated from 

 Sertularia. In all these cases the difference depends on the formation in the female of the remark- 

 able marsupial chamber whose structure has been already described (see p. 52). 



It will also be borne in mind that, in those species which develop an acrocyst on the 

 summit of the gonangium, this body is formed only in the female ; while it is on the female 

 gonangium alone of Halecium halecinum that the little geminate hydranth already described 

 (p. 58) is produced ; and to these cases we may also add the difference presented by the male 

 and female meconidia in Gonothyrea Loveni (see p. 5G). 



Among the gymnoblastic hydroids, also, certain differences may be occasionally observed 

 between the male and female. Thus, the tentaculoid tubercles which, in certain Tulularice, crown 

 the gonophore are in some species more fully developed in the female than in the male ; but the most 

 striking difference is fouiul in the genus Eudendriuin, whose male gonophores are situated in a 

 verticil on the body of the hydranth, and present the remarkable polythalamic condition already 

 described, while the female gonophores originate irregularly for some distance backwards on the 

 branch, and are always monothalamie (see Pis. XIII and XIV). This difference between the 

 male and female gonophores in Eudendrium struck Cavolini long before the presence of a male 

 element in the Hydroida was suspected, and led him to suppose that Eudendrium reproduced 



cells) unitiug to form a protecting capsule or germeu ; whicli metamorpliosis is exactly comparable with 

 that which occurs in the reproductive organs of flowering plants, in which the floral bud (normally a 

 branch clothed with spirally arranged leaves) is constituted through the contraction of the a.\is and the 

 whoiling of the (individual) appendages borne on that axis, and by their transformation into the several 

 parts of the flower (reproductive organs)." 



The theory, however, involved in the above statement, attractive though it be, is contradicted by 

 the actual development of the parts in question. When Forbes wrote, so little was known of the struc- 

 ture and development of the Hydroida, that this accomplished and lamented naturalist may well be 

 excused if some parts of his very suggestive paper have refused to stand the test of subsequent research. 



^ It is apparently the m^le gonangia which Ellis has figured iu his description of this species. 



