2-26 ANATO:\IY OF SPECIAL FOR^IS. 



consist of a simple irranulnr protophisiii, wliicli in its power of extension nntl retraction at once 

 recalls the pseuclopodia of a rliizo[)0(l. 



DlCORTNE CONFKllTA. 



Plate YIII. 



This hydroid may be found investing the surface of various univalve shells in the form of a 

 dense moss-like growth. It aflbrds the only example we yet know of a free locomotive 

 sporosac. 



27ie Hydrajj/zj/foii and the Jhidranllis. — The hydropliyton (fig. 1) consists of branched stems 

 with occasionally some simple ones, all arising from a creeping rctiform hydrorhiza, the whole 

 invested by a rather coarse perisarc. 



The hydranths (fig. 1) are fusiform, with a single circlet of filiform tentacles surrounding the 

 base of a conical hypostome. 



The Gonop/iorcs. — It is the gonophorcs, however, which confer upon liicorync a special 

 interest, and eminently distinguish it from all other known hydroids. They are borne on true 

 blastostyles (fig. 1), which spring either from the hydrocaulus or from the hydrorhiza, and which 

 represent hydranths modified by the complete suppression of both tentacles and mouth. The 

 gonophores are in the form of an oval sac, and are densely crowded on the sides of the blas- 

 tostyle. The male and female clusters entirely resemble one another in external form ; they 

 are always borne on separate stems, but I have occasionally met with both male and female 

 stems in the same colony. While still attached to the blastostyle the gonophores (fig. 5) may be 

 seen to consist of an external sac (ectotheca), within which is a second sac (endotheca). Within 

 this second sac are the ova or spernuitozoa, while a long simple spadix occupies its axis. By 

 'carefully adjusted compression it will also be seen that just behind the proximal end of the 

 spadix two tentacula-like processes are given off. These run forward between the ectothecal 

 and endothecal sacs until they reach a point nearly half way between the proximal and distal ends 

 of the gonophorc. 



On attaining maturity the endothecal sac and its contents arc ready to liberate themselves, 

 and the sac accordingly becomes detached from the summit of its peduncle along a defined line 

 which lies just behind the roots of the tentacular appendages, and, carrying these appendages 

 along with it, breaks through the endotheca and becomes a free zooid in the open sea (figs. 3, 4, 

 G), leaving the torn ectotheca behind it attached to the summit of the peduncle. 



The planoblast thus set at liberty is a sexual zooid of an exceedingly 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 previoiisly turned forward as they lay impacted between 

 the ectotheca and endotheca of the gonophorc, and which, now becoming extended to at least 

 twice their former length, diverge from the proximal extremity of the liberated zooid. 



This zooid is thus a free sporosac. It is an oval sac with a pair of tentacles diverging from 

 its proximal extremity. It is completely covered with vibratile cilia, which extend even to the 

 extremities of the two tentacles, and by their aid it swims actively in the surrounding water. A 

 long spadix runs through the entire axis of the sac impinging on its sununit and there remaining . 



