Prof. Alhnan on the Hydroid Zoophytes. 



169 



The gonophores of Dicoryne are, as I have already described 

 (toe. cit.), borne upon peculiar polypes destitute of tcntacula 

 and mouth. They are of an oval shape, shortly pedunculated, 

 and grouped in clusters of from about 10 to 20 or more in each. 

 The male and female are similar in form. They consist of an 

 external sac (ectotheca*), within which is a second sac (endotheca) 

 containing the ova or spermatozoa, and with a long simple 

 spadix occupying its axis. Immediately behind the endotheca, 

 and between it and the ectotheca, two tentaculum-like processes 

 are given off exactly opposite to one another; and these, con- 

 tinuing to lie between the two membranes, run forward for some 

 distance towards the summit of the gonophore. No mesotheca 

 is developed. 



When the gonophore attains maturity, the ectotheca bursts, 

 and the endotheca! sac, separating itself from the peduncle just 

 behind the two tentacular processes, escapes into the surround- 

 ing water, leaving the torn ectotheca attached to the summit of 

 the peduncle. 



The body thus set at liberty is a sexual zooid of an exceed- 

 ingly singular kind, and, so far as we yet know, without any 

 exact parallel. Immediately on acquiring its freedom it throws 

 back its two tentacles, which had been previously turned forward 

 as they lay impacted between the ectotheca and endotheca of 

 the gonophore, and which now becom- 

 ing extended to two or three times their 

 former length, diverge from the poste- 

 rior extremity of the liberated zooid. 

 Its form is that of an oval, pointed at 

 one end (anteriorly) and truncated at 

 the other (posteriorly), so as somewhat 

 to resemble the front view of a mitre, 

 and with the two tentacles diverging 

 from the truncated extremity. Ante- 

 riorly it consists of a spacious sac, which 

 is occupied by the generative elements 

 surrounding a long spadix which ex- 

 tends through the entire axis of the sac. 

 impinging upon its summit and there 

 remaining adherent to its walls ; while 

 posteriorly the zooid becomes more con- 

 tracted, and into this part the sac does 

 not extend. The whole body is covered with long cilia, which 



* For an explanation of the terminology employed in the present paper, 

 I must refer to my " Note on the Structure and Terminology of the 

 Reproductive System in the Corynidse and Sertulariadae," published in 

 the ' Annals of Natural History ' for July 18(i0. 



