GEMMARIA IMPLEXA. 223 



iiieiitioned the existence of a well-developed nioutli in the American species described hj him. 

 A fibrillated layer is developed, as in the hydranths and the spiral zooids, between the ectoderm 

 tmd endoderm, throughont the whole length of the blastostyle. 



The gonophorcs are simple sporosacs and are borne near the distal extremity of tiic 

 blastostyle, usually in a closely aggregated cluster. Tliey are of an oval form, with a well- 

 tleveloped spadix. Tlieir walls (fig. G) consist of tw-o membranes, an internal thin membrane 

 (endotheca), in which no evident structure can be detected, and au external (ectotheca), with a 

 <listinctly cellular structure and containing abundance of thread-cells. No mesotheca is developed. 



The ova are invested by a delicate but distinct vitellary membrane. They contain a very 

 large and evident germinal vesicle, within which a nucleated germinal spot may be easily detected 

 (fig. G). Each colony is absolutely unisexual. 



Quatrefages mentions the occurrence of bodies which he regards as eggs, and which he has 

 seen escaping from the ccenosarc when this had been torn muler the microscope. I have never 

 noticed those bodies, and am unable to form any opinion as to their nature. We may compare 

 the observation of Quatrefages with Cavolini's account of egg-like bodies which he also found 

 escaping from the ccenosarcal cavity of his Ser///laria jjarasifica. 



The hydranths, blastostyles, and spiral zooids arc constant, but besides these Dr. Wright 

 has described certain long filiform bodies which, like the spiral zooids, occur near the margin of 

 the colony. He calls them " tentacular polypes," and regards them as constant, and employed 

 by the colony in the prehension of food. They are, however, by no means constant, nor are they 

 even usually present, and I cannot consider them as otherwise than abnormal. They arc probably 

 malformations which some of the spiral zooids or some of the hydranths situated close to the margin 

 have uiulergone, from having been there exposed to conditions unfavorable to their development. 



Dr. AA'right also describes as an essential part of the colony certain sporosacs similar to 

 those which are borne on the blastostyles, but which, instead of being so borne, spring directly 

 from the ccenosarc. I think that there is here some error. I have never met with these cceno- 

 sarcal sporosacs, while in a specimen given to me as affording an example of their occurrence I 

 could find nothing of the kind, the sporosacs which on a superficial view had the appearance of 

 arising directly from the ccenosarc proving on close examination to arise from the sides of very 

 short blastostvles. 



GeM.MARIA IMPLEXA. 



Plate VII. 



Tliis beautiful httle hydroid may be found on the under side of stones at extreme low- 

 water mark, or it may be dredged up from the Coralline Zone, where it grows over the surface of 

 old shells. It is well deserving of a special study, for its medusa is in many respects one of the 

 most remarkable in the whole range of planoblasts. 



T/ie iri/droj}//?/(on.— The hydrophyton is but little developed in proportion to the hydranth, 

 jind consists of short stems, very rarely sending off a branch, and with a creeping filiform 

 hydrorhiza (figs. 1, -2)} 



^ A variety witli the liydrocaulus more developed and more decidedly branched would scera 

 occasionallv to occur. 



