INTRODUCTION. Ill 



the coenosarc, to a greater or less extent, in all (?) the fixed 

 Hydroida. It varies in the degree of development, some- 

 times investing the whole of the cosnosarc, and sometimes 

 only a small portion of it. It usually forms a solid cover- 

 ing, but in some cases is a mere membranous film. 



HYDROTHECA or CALYCLE. The chitinous receptacle in 

 which the polypites are lodged in one of the Hydroid sub- 

 orders ( Thecaphora) . The term cell has been commonly 

 applied to this portion of structure ; but as this word is 

 now generally used in physiological science with a totally 

 different meaning, it will be more convenient to dispense 

 with it here. 



GONOPHORE. The bud in which the reproductive ele- 

 ments are formed. It consists of an external envelope 

 (ectotheca), enclosing either a fixed generative sac, between 

 the walls of which the ova and spermatozoa are developed, 

 or a free sexual zooid*. 



GONOZOOID. The sexual zooid, whether fixed or free ; 

 the gonophore minus the ectotheca. In some genera 

 (e. g. Clavatella and Corymorpha) the gonozooids are 

 destitute of an ectothecal covering at all stages of their 

 development. 



* A somewhat different meaning is given to the term gonophore from that 

 which it bears in the works of Prof. Allman, by whom it was introduced. A 

 little uncertainty, it seems to me, connects itself with his use of the word. 

 He has denned the gonophores to be " certain buds of a peculiar structure, 

 destined for the formation and protection of the generative elements." Of 

 these buds an ectotheque or protective envelope is, in a large proportion of 

 cases, an essential part ; and the contained sexual zooid, when free, makes its 

 escape from this outer sac, which then withers away. Now, as employed by 

 Allman, the gonophore is sometimes the reproductive bud with its ectotheca, 

 and sometimes the contained zooid, whether fixed or free. To me the gono- 

 phore is the whole bud, and the sexual zooid developed in it, whether as a 

 fixed sac or a floating polypite, is the gonosooid. 



a2 



