Common Hydra. 255 



the sea-fir, Sertularxa ahietina {iov which, see Description of Prepara- 

 tion 48, p. 160), with the free 'Medusae,' would induce us to speak of 

 them as 'individuals.' 



f. H^^drorhiza^ forming the single point of attachment of the 

 animal, and supposed by some to be perforated so as to give 

 passage outwards to certain secretions from the body cavity, 

 the mouth being undoubtedly the channel by which the 

 refuse of the ingested alimentary matter is ejected. The 

 external layer of integument covering the adhering disc, is 

 made up of sub-cylindrical cells, larger and more closely set 

 than those on the rest of the polype- stem. 



For the histology of the Hydridae, see Huxley, Miiller's Archiv., 

 1851, p. 381 j Oceanic Hydrozoa, 1859, p. i; Leydig, M'Jdler's 

 Archiv., 1854, p. 276; Professor Reichert, Monatsbericht der 

 Akademie der Wissenschaf en zu Berlin, July, j866, translated 

 in the Annals and Magazine of Natural History for January, 

 1867, p. 54. For the intermediate excretionary layer, see 

 Claus, Zeitsehrift fiir Wiss. Zoologie, x.. 1 860, p. 300. See also 

 Kolliker, Icones Histiologicae, ii. Abtheilung, pp. 88, 100, 1 866 ; 

 Leuckart, Archiv. fiir Naturgeschichte, 1 854,1. 369 ; Leydig, I. c. 



For the discussions as to the individuality of the various Zooids, see 

 Claus, I. c, pp. 326-329 ; Gegenbaur, Vergleichende Anatomic, 

 pp. 94-103, where, as also in V. Carus^ Icones Zootomicae, p. ii., 

 exceedingly instructive semidiagrammatic figures of those 

 organisms in the Hydrozoa are given. See also Hincks, His- 

 tory of British Hydroid Zoophytes, pp. xxxv-xxxix ; Leuckart, 

 Polymorphismus der Individuen oder die Erscheinungen der 

 Arbeitstheilung in der Natur., 1851. 



FiGUEE 8. 



Holotnchous Infusorium {Prorodon Teres), from Stein, in V. Carus' Icones 

 Zootomicae, Taf. i,, fig. 27. 



We see its armed oral inlet at one pole of the body, its con- 

 tractile vesicle at the other, the cilia wdiich in the sub-order 

 Holotricha clothe the entire surface with a uniform covering, the 

 sexual organs, and the globular masses of nutriment, which having 

 been ingested by the mouth have been distributed throughout the 

 parenchyma of the body. 



