32 



MORPHOLOGY. 



element in the character of the species, and tlie study of no one species of hydroid can be regarded 

 as complete unless it embrace both trophosome and gonosome. Since, however, in many cases we 

 are acquainted with only the free gonophore, or the free blastocheme, not having yet discovered 

 the trophosome to which it belongs, while in other cases the trophosome alone is known 

 to us, we have been in the habit of treating such instances without regard to the missing 

 zooids, and as if they afforded examples of independent species ; but it must never be forgotten 

 that the data on which we thus assign to them the rank of determinate species, or genera, 

 are insufficient for the purposes of a philosophic system : such genera and species must be 

 regarded as purely provisional ; for the zoologist is no more justified in accepting such incom- 

 plete characters as sufficient for the accurate determination of his hydroid, than would the 

 botanist be in regarding the flower alone on the one hand, or the root, stem, and leaves alone, on 

 the other, as affording characters sufficient for the definition of any flowering plant whose exact 

 determination he woidd attempt. 



The gonosome is that part of the hydroid which presents the most marked variation among 

 the different members of the group, and it is here that we find most distinctly manifested those 

 beautiful gradations of form which, by throwing light not only on one another, but on forms 

 belonging to other groups of the Hi/drozoa, possess for the morphological student a profound 

 significance. It will therefore be necessary to enter into a more detailed examination of the 

 gonosome in its various modifications aniono; the Hydroida. 



Fig. 7. 





Adelocodonic gonophore of llydract- 

 nia echinata. 



a, Spadix surrounded by the ova, the 

 whole enveloped by the perigonium, 

 which here consists of J, endotheca, and 

 c, ectotheca, the mesotheca being absent ; 

 rf, communication of the cavity of the 

 spadix with that of the blastostyle. 



The Adelocodonic Gonophore {Sporosac). 



The adelocodonic gonophore (woodcut, fig. 7) is a sac- 

 like body, which presents the following parts : 



1. An internal hollow process, the spadix [a], which occu- 

 pies the axis, and whose cavity is in communication with the 

 general somatic cavity of the hydrosome. 



2. The generative elements, ova or spermatozoa, which are 

 developed round the spadix. 



3. A sac [peric/onium) which surrounds the generative 

 elements, and retains them in their place. This sac may be simple 

 or its walls may present two or even three layers, in which last 

 case it will consist of (1) an internal layer [endotheca], which lies 

 immediately on the generative elements; (2) a middle layer 

 {mesotheca) ; nwA. {i) a.\\ external layer {ecfotheca). The meso- 

 theca, moreover, may have a system of canals developed in it. 



Of the adelocodonic gonophore we have examples in 

 such forms as the so-called generative sacs of Clava, Ilydractinia, 

 Eudendrium, &c. 



3. The Phanerocodonic Gonophore {Gonoclieme. Sexual Medusa). 

 The phanerocodonic gonophore (woodcuts, figs. S and 17) is bell-shaped, and in its most 

 developed form presents the following parts : 



1. A central tubular body, the manubrium [a), which carries a mouth (/) at one extremity. 



2. The generative elements [b), which are developed between the inner and outer membranes 

 (endoderiu and ectoderm), which compose the walls of the manubrium. 



