POLYPS. 69 



organs may exist at the same time, and in variable number, in one 

 and the same individual. And many other Polyps are also herma- 

 phrodite. In others again the sexes are separate : whether both male 

 and female individuals occur on one and the same stem (Moncecia, 

 as in Plants), or one Polypstock bears only males, another only 

 females (Dicecia). The last is the case of Veretillum. In the 

 Bryozoa, Monoscia appears to prevail universally, yet so that (to 

 judge from the investigations of NORDMANN in Tendra zostericola, 

 and of VAN BENEDEN in Alcyonella) the cells which contain Polyps 

 with eggs are more numerous than those with spermatozoa. These 

 peculiar constituents of the seed (vid. above, p. 43), of which the 

 motions are so striking under the microscope, have, of late years, 

 caused the important discovery of the sexual propagation of Polyps ; 

 but for them, ovaries alone would now, as twenty years ago, be 

 ascribed to this class, especially as the seed-secreting organs (testes) 

 are not to be distinguished in it, as to external appearance, from 

 those that prepare the germs (ovaria) 1 . In those Anthozoa that 

 have, like the Actinia?, a cavity of the body distinct from that of 

 the stomach, they are situated between or upon the partitions that 

 divide that cavity into cells (see above, p. 67). In Sertularia and 

 Campanularia most of the Polyps are without sex, whilst cells with 

 ova are developed in the axillae of the branches. 



Propagation by spontaneous division does not occur in most 

 Polyps. In Caryophylla there is a complete longitudinal fission, 

 occasioning the dichotomous form of the Polypary, since two Polyps 

 come from one, four from two, &c. If the longitudinal fission be 

 incomplete, cells of irregular form arise, as in Mceandrma. 



In most Polyps the power of reproduction is very great. TREM- 

 BLEY'S experiments on the fresh-water Polyp are well known : he 

 divided them longitudinally and transversely, and every piece 

 formed a new animal 2 . ROESEL found that even the tentacles or 



1 Such is the case also in Mollusca, nay even in some fishes ; and in general the 

 sexual organs in the animal kingdom possess a similarity in the two sexes, which was 

 observed by the ancients, and occasioned many fanciful appellations and comparisons. 



2 Hence LINN^US borrowed the name Hydra for this animal genus, from a com- 

 parison with the Hydra of mythology : 



. . . ab ipso 

 Ducit opes animumque ferro. 



HORAT. Od. iv. 60. 



