14 ANTHOZOA IIYDROIDA. 



lowest tribe of animals;"* but no direct observation has con- 

 firmed this explanation, which, it will be observed, is founded 

 on analogy only ; and it has this in opposition that the non- 

 existence of cilia in the external organs of the zoophytes in 

 question has been distinctly proved. As to the purpose of 

 the circulation in the animaPs economy, it appears, from the 

 experiments of Mr. Lister, " to be the great agent in absorp- 

 tion, and to perform a prominent part in the obscure pro- 

 cesses of growth ; and its flow into the stomach of the polypi 

 seems to indicate that in the very simple structure of this 

 family it acts also as a solvent of the food. The particles 

 carried by it," continues Mr. Lister, " present an analogy to 

 those of the blood in the higher animals on one side, and of 

 the sap of vegetables on the other. Some of them appear to 

 be derived from the digested food, and others from the melt- 

 ing down of parts absorbed ; but it would be highly inter- 

 esting to ascertain distinctly how they are produced, and 

 what is the office they perform, as well as the true character 

 of their remarkable activity and seemingly spontaneous mo- 

 tions ; for the hypothesis of their individual vitality is too 

 startling to be adopted without good evidence."^ 



It had been so long agreed that there were no sexes amongst 

 the Hydroid zoophytes that naturalists were rather startled 

 when Ehrenberg announced the contrary to be the fact. This 

 celebrated micrographist asserts that in the Hydra there is a 

 periodical development of sexual organs of two kinds, small 

 sacs to wit at the oral extremity, which contain seminal 

 animalcules, and a series of cells at the posterior part of the 

 body in which the ova are produced. The animal is conse- 

 quently hermaphroditical, but "sometimes one individual 

 Hydra develops only the male cysts, or sperm-vesicles ; some- 

 times only the female ones, or ovi-sacs.' 1 ]: Ehrenberg ex- 

 tended this view to other polypes : the individuals in the per- 

 manent cells of the Sertulariadee were considered to be sterile 

 or male, while the almost amorphous or imperfectly developed 

 individuals in the deciduous vesicles, which appear at certain 



* Outlines of Comp. Anat. p. 430. 



+ Phil. Trans. 1834, p. 377. 



J Owen's Lectures, p. 85 ; Lancet, No. 871, p. 225. 



