4 THE OCEANIC HYDROZOA. 



The transverse section of the filiform and tree-like ccenosarcs is usually nearly circular, 

 but in some FhysophoridcE [Forskalia, e. g.) it is said to be reniform, from the presence 

 of a deep longitudinal groove on one side. 



The ccenosarc, as has been stated, always contains a cavity filled with nutritive fluid — 

 the somatic cavity. The endoderm lining this cavity is in many, if not in all, Corynida and 

 SertulariadcB, provided with cilia, whose motions are, in many cases, so directed as to give 

 rise to currents in ojjposite directions on opposite sides of the cavity. The like phenomenon 

 has been observed in his " Ay alma rubra'' {Halistemma, mild) by Vogt (p. 64); and cilia 

 have been noticed on the endoderm of the ccenosarc of many Physophoridm. I have 

 observed them in Physalia, Velella, and Ithizophysa. Gegenbaur (p. 42) saw them in Bhizopliysa 

 and Apolemia. Kolliker, however, denies the existence of cilia on the endoderm of the 

 ccenosarc in Forskalia, Agalma, and Ilalistemma and is silent with respect to the other species 

 of PhysophoridcB which he describes, except Velella and Porpita, in which the ciliation is 

 distinctly mentioned. Leuckart (Z. U., p. 4) agrees with Kolliker in denying cilia to the first- 

 named genera. 



Will (' Horae,' p. 78) expressly afl&rms that the inner wall of the ccenosarc of Biphyes 

 exhibits as lively ciliary motion as that of the polypites ; but I could never verify this state- 

 ment, nor find cilia on the endoderm of the coenosarc (except at its proximal end) in any of the 

 Cahjcophoridce . On the contrary, I have distinctly observed and noted the fact, that solid 

 particles, which, so long as they are in the cavity of the polypite, exhibit a lively rotatory 

 motion, impressed upon them by its cilia, lose that rotation the moment they pass through 

 the pyloric valve into the somatic cavity, and then either remain stationary, or are forced sud- 

 denly along it, backwards or forwards, by the contractions of its walls or of the attached organs. 

 Leuckart and Kolliker have been equally unable to find cilia in any part of the somatic 

 cavity of the Calycophorida, except its proximal dilated end ; but Gegenbaur aflirms that it is 

 ciliated throughout in his Biphyes gracilis {r=^ B. Sieholdii of Kolliker, in which, however, 

 that observer distinctly states no cilia exist), and in Praya maxima (' Beitrage,' p. 21). 



In the Lucernariadm and Medusidce, the inner surface of the endoderm is, so far as I have 

 seen, everywhere cihated. 



The Appendages. 



Before attempting to describe the structure and relations of the manifold appendages of 

 the Hydrozoa, it will be necessary to determine the corresponding ends 'of the hydrosoma 

 throughout the series of forms, a task which is not quite so easy as it may at first appear 

 to be. The Hydra is fixed by one end, which is ordinarily lower than the other, and may 

 be regarded as the base ; and the end which answers to this is inferior, or basal, in 

 air the Corynida and Sertulariada. But the Calycojjhoridce swim with the corresponding end 

 upwards or forwards. The Physophorida float with it upwards. The Luceniariada have it 

 sometimes upwards and sometimes downwards. It would be impossible, therefore, to 

 designate the corresponding ends by the terms " upper and lower" throughout the series 

 without great risk of confusion. Under these circumstances I think it will be best to discard 

 such phraseology, and to employ the terms " proximal" and " distal" for the same object. 



