14 THE OCEANIC HYDROZOA. 



inclined, not without a great show of reason,^ to regard them as organs of prehension and 

 touch, to which may perhaps he added excretory and respiratory functions. 



The hydrocysts are always pyriform sacs, composed of the ectoderm and endoderm, 

 shut at their apical or distal ends, where they are commonly provided with large thread-cells, 

 Init, at their proximal ends, in free communication -with the somatic cavity. Like the polypites, 

 they usually give origin to a tentacular appendage. But this is always simple and filiform, 

 and the hydrocysts further differ from the mere polypites in their closed apices and in the 

 general absence of villi. The latter, however, exist in a rudimentary state in the hydrocysts 

 of Jpolemia, according to Leuckart (Z. N. K., 70), and I have seen them in the closed 

 sacs, which appear to be hydrocysts, of PUysalia (PI. X). These bodies are not found in 

 Velella or Porpita, and I must confess I am very doubtful, whether the structures to which I have 

 just referred in Physalia are other than young polypites. In the StejjJianomiache they 

 are attached to the coenosarc, between the polypites, and are usually in more or less close 

 relation with the reproductive organs. In Phi/sopJiora (PI. VIII) a circlet of large hydrocysts 

 is interposed between the nectocalyces and the polypites, and in Jpolenda, according 

 to Kcilliker, Gegenbaur, and Leuckart, a group of them surrounds the base of each polypite, 

 while solitary ones are, in addition, interspersed between the nectocalyces. 



4. Hydrotheca, and 5. Ilydroplii/Uia. 



Many liydrozoa possess appendages whose only function would appear to be to serve as 

 a protection to other parts of the organism, more especially the polypites. Of these organs 

 there are two very distinct kinds, the one peculiarly characteristic of the Serlulariada, 

 the other found only among the Bipliydce and Pliysoplioridcs. The former, or Jiydrotlwca, 

 are what are commonly termed " polype cells." They are cup-like receptacles, entirely 

 composed of the cuticular layer of the ectoderm, and consequently contain no diverticulum 

 of the somatic cavity at any period of their existence. They are primarily developed 

 from the whole outer coat of a budding polypite. 



For the second class of protective organs, or InjdrdphyUia, there is no generally accepted 

 English name. I have termed them '•' bracts " in my earlier memoirs, and the Germans 

 call them " deck-stucke." 



The hydrophyllia differ entirely from the hydrothecse, for they always contain a diverticulum 

 of the somatic cavity, or phyllocyst, and are consequently composed of both ectoderm and 

 endoderm, though their principal mass is furnished by the ectoderm. Again, they always 

 commence their existence by budding from a limited segment of the coenosarc, or of the pedicle 

 of a polypite. In the CalycopJioridce, they are attached to the pedicles of the polypites, 

 which they eventually surround more or less completely (PI. V). They are either thin and 

 foliaceous, as in Bipliyes, or thick and facetted externally, as in Ahyla. Their form and 

 arrangement vary greatly in the PJiysophoridce. In ForsJcalia, numbers of them are developed 

 from the pedicles of the polypites, as well as from the coenosarc, while in Agalma (PI. VI) they 



^ See Kolliker, p. 21 ; Leuckart, Z. U., p. 17 ; aucl, again, Z. N. K., p. 69, where the strong 

 resemblance of these bodies to polypites is fully admitted. 



