208 ANATOMY OF SPECIAL FORMS. 



elements soon make their appearance Ijetween the endodermal and ectodermal layers, which thus 

 l)econie separated from one another, and the ectoderm becomes an endotheca. 



The generative mass in the female (fig. 11,/) constitutes a cellular plasma, into which the 

 spadix is plunged and from which portions are successively detached. These detached portions 

 {(j) lie loose in the cavity of the gonophore after the disappearance of the endotheca. They consist 

 of a mass of clear spherical cells, many having within them a brood of secondary cells (fig. 12). 

 They become developed into embryos, and though I have never succeeded in detecting in them 

 a germinal vesicle or witnessed in them the phenomenon of yolk-cleavage, they must, for reasons 

 stated in a former part of this work (see p. 90, &c.), be regarded as true ova in various stages of 

 development. 



As already mentioned, the ovum becomes developed, not into a ciliated planula, but into an 

 actinula. The various stages of this development and the structure of the actinula are repre- 

 sented in figs. 13 — 10, and have been already fully described (see above, p. 90, &c.). 



In the male the spermatogenous tissue also forms a mass (fig. 8, /) into the midst of which 

 the spadix is plunged. It is pi'oduced, like the generative mass in the female, between the 

 endoderm and ectoderm of the central process, so that the ectoderm becomes separated as an 

 endotheca (e), which continues longer apparent than in the female. As the spermatogenous mass 

 approaches maturity it presents the appearance of radiating striae, and after the rupture of the 

 endotheca the free spermatozoa escape as active caudate corpuscles (fig. 9) through the aperture of 

 the gonophore. 



CORYMORPHA NUTANS. 



Plate XIX. 



This beautiful hydroid is eminently conspicuous by the great size of the solitary hydranth 

 with its dense clusters of medusae, and by the thick fleshy stem destitute of that firm chitinous 

 ])erisarc wjiich forms a strong protective covering for others, while in its internal structure it is 

 further distinguished by well-marked and ])ecuhar features. Whether om- attention be directed 

 to its trophosome or to its gonosome, we cannot fail to recognise in Corpnorjjha a special confor- 

 mation which marks it out as a very distinct hydroid type among its co-ordinal associates. 



The Hydrophjton. — The hydrocaulus (figs. 1, 2), which is always solitary, attains a height of 

 from two to three inches, while it varies in thickness from its distal to its proximal end, usually 

 attaining, in its thickest part, which is situated a little above its attached extremity, a diameter 

 of about two lines, and thence rapidly tapering downwards to a blunt point, which is plunged into 

 tlie sandy sea-bottom. The hydrocaulus is here a little above its proximal extremity bent nearly 

 at right angles on itself. The place of the perisarc is taken on it by an exceedingly thin colourless 

 and structureless film, which without careful examination would quite escape detection, except 

 towards the proximal extremity, where it is separated by a considerable interval from the ectoderm 

 of the stem, and thus becomes at this part sufficiently obvious. 



Towards the proximal extremity fleshy conical processes arc developed from the sides of the 

 stem in longitudinal rows (fig. 1, a), while still lower down the stem gives off all round great 

 numbers of very much attenuated and elongated filaments {h, b). 



Even by the naked eye the stem will be seen to be traversed from end to end by longitudinal 



