THE GONOSOME. 55 



Iiydi-oid had attached itself. Each gonangiura in the female contains a single sporosac with 

 a single ovum ; and this ovum, after a time, becomes extra-capsular in order to undergo within 

 an acrocyst some of the earlier stages of its development.' 



But tiiere is, perhaps, no modification of the gonosome more interesting than that which we 

 meet with in Gonothijraa Loveni, Allm., and at least one other allied species. In this calypto- 

 blastic hydroid there are borne upon the summit of the gonangium, and altogether external to 

 its cavity, certain very peculiar gonophores, which convey the impression of small, fixed, 

 imperfectly dcvcloiiutl medusc^ (woodcut, fig. 2S). It was to these extracapsular gonophores 



surface of the coenosarc to the inner surface of the chitinous pciidcrm, and which these tubes 

 certainly resemble when they become more or less atrophied and adherent to the walls of the 

 gonangium. They are also described and figured by Lindstrom in a paper " On the Development of 

 Sertulariapumi/a" (' Oefversigt af Kongl. Vetenskap-Akademiens FiJrhandlingar,' 1855). 



' As the nature of Coppinia arcta has been hitherto very imperfectly understood a more detailed 

 description of it may here be given. 



The hydrosoma forms small sponge-like masses on the stems of the larger hydroids, and is espe- 

 cially abundant on Seriularia abietina and Pimmlaria falcata from deep water. 



Even on a superficial inspection it may be seen to consist of two distinct portions. Of these, one 

 constitutes a continuous encrusting base, and the other consists of curved cylindrical tubes which 

 project from the free surface of the base. In each of these tubes is contained an essertile and retractile 

 hydrauth. The tubes are thus true hydrothec*. The hydranths are conspicuous by their fine lemon- 

 yellow colour, and are furnished with a verticil of filiform tentacles disposed round the base of a 

 short conical hypostome. They are, however, often imperfect and apparently destitute of mouth and 

 tentacles. 



The encrusting base which forms the most remarkable part of the hydroid has never yet been 

 correctly described. 



The liydrothccal tubes can be traced through it to its attached surface, while vertical and 

 transverse sections show that the rest of the crust is mainly composed of vertical chitinous 

 tubes rendered polygonal by mutual pressure. They adhere to one another by their sides, and 

 each, as had been long ago shown by Dalycll, opens on the free surface of the crust by a small circular 

 aperture. 



These tubes are true gonangia ; within each is a solitary sporosac which seems to have originally 

 budded from a blastostyle, which soon, however, becomes suppressed by the growing sporosac. A 

 sufficiently obvious spadix may be recognised in the sporosac which contains a single large lemon- 

 yellow ovum, in whose earlier stages there may be seen a distinct germinal vesicle, while the place of 

 the germinal spot is taken by numerous clear spherical bodies which disappear in a few seconds after 

 the ovum is pressed out of the sporosac and exposed to the action of the surrounding water. 



Segmentation commences while the ovum is still within the gonangium, and the ovum becomes 

 thereby converted into a mass so plastic that it allows of its being forced through the small aperture 

 in the summit of the gonangium. In its exit it carries out with it a hernial extension of the attenu- 

 ated walls of the sporosac, which thus form for it an acrocyst in which it is still for some time confined. 

 It ultimately, by the rupture of the acrocyst, escapes as a planula into the surrounding water. The 

 planula and its development into a hydranth enclosed in a chitinous tube have already been observed 

 by Daly ell. 



Both hydrotheca; and gonangia spring from an adherent retiform hydrorhiza without the inter- 

 vention of a hydrocaulus. 



A knowledge of the structure of Coppinia will enable us to give a more correct generic 



