162 Descriptions of Preparations. 



present species) — the Hydrozoa differ widely and essentially from the 

 Polj^zoa. It may be further added that it is very usual in Polyzoa 

 to have each cell cut off by a diaphragm or septum^ into the forma- 

 tion of which both ectocyst and ehdocyst enter, from continuity 

 with the rest of the colony ; whilst it is only in the few cases such 

 as Hydra, Corymorpha, Vorticlava^ Myriothela^ in which the hy- 

 drosoma consists of but a single polypite, that such independence 

 is attained to in the Hydroid Zoophytes. In the form of ' Poly- 

 morphismus/ which the ^medusoid bud^ presented to us in the 

 'gonophore^ of the Sea-Fir exemplifies^ we have the connecting 

 link between the distinct testis and ovary of the Hydra, (for which 

 see pi. xii., fig. J, and description), and the free sexual zooids 

 known as ' Medusae,' and exemplified in genera as nearly akin to 

 the Sea-Fir as the Campanularia gelatinosa. And, as has been 

 well observed, a study of such histories as those of the various 

 modes of development of this class of Coelenterata, shows how 

 impossible it may become to draw sharp lines of distinction be- 

 tween individual animals or zooids and simple organs on the one 

 hand; and on the other between asexual generation and simple 

 growth. 



For a monograph of the British Hydroidea Diplomorpha, V. Carus, 

 see Hincks, History of the British Hydroid Zoophytes, 1868, 

 where a general account of their structure and life-history is 

 given in the Introduction, pp. i.-lxv. For the Siphonophora, 

 see Professor Huxley, Oceanic Hydrozoa, Ray Society, 1859. 

 For the Hydrozoa generally, see Professor Huxley, Medical 

 Times and Gazette, June 7, 1856, p. ^60^. 



For an account of the particular species to which the specimen 

 here described belongs, see Hincks, I. c, i,, 226. 



For an account of the development of the Sertularia ciipressoides, 

 see Van Beueden, Recherches sur THistoire Naturelle des 

 Polypes, 1866, pp. 179-184, pi. xvi., where what is spoken 

 of above as a ' marsupial sac,^ and described and figured as 

 such by Hincks, I. <?., p. xvi., is figured at fig. 3, but spoken of, 

 p. 180, as a ^hernie au bout de la capsule/ and for the de- 

 velopment of the Hydroidea generally, see Allman, British 

 Assoc. Eep. for 1863, p. 351. 



For a short account of the Histology of this class, see Professor 



