32 STRUCTURE AND PHYSIOLOGY 



and which, though commonly present, is yet not necessary to 

 the existence of a zoophyte. To the structure and functions 

 of the former I hmit myself in this chapter ; and should the 

 reader find the outhne given in relation to some of the families 

 too slight and sketchy, I may advertise him that he will find 

 it filled up with greater detail in the observations which it is in- 

 tended to prefix to each separate order. 



The description usually given of the structure and functions 

 of polypes in general has been derived principally from an ex- 

 amination of the Hydra— a naked species which inhabits ponds 

 and ditches. A polype is thus represented as being a somewhat 

 globular or cylindrical body of small size, of a homogeneous gela- 

 tinous consistence, and very contractile, in the centre of which 

 there is excavated a cavity ^for the reception and digestion of its 

 food. The aperture to this cavity is placed on the upper disk 

 of the body, and is encircled by one or two series of filaments 

 or tentacula which are used to capture the necessary prey, and 

 bring it within reach of the lips ; while the opposite end serves 

 the purpose of a sucker to fix the creature to its site, or being- 

 prolonged like a thread down the hollow sheath, to connect it 

 with its fellow-polypes of the same polypidom, which by this 

 means become compound animals, " the whole of whose parts 

 are animated by one common principle of life and growth." 

 There are no organs of sense, no limbs appropriate to locomo- 

 tion, no circulating vessels, no nerves, nor lungs, nor gills, no 

 chylopoetick viscera, nor intestine, for there is " but one con- 

 duit both for purgation of their excrements, and reception of 

 their sustenance ;" and when to these negations there is to be 

 added the want of generative organs, a being of simpler organi- 

 zation than the polype can scarcely be conceived ; and, perhaps, 

 it is actually the simplest, for the infusory animalcules which 

 had been placed underneath them in the scale of organization, 

 are now known to possess a much more complex structure. 



Such is the idea of a polype we obtain from the writings of 

 Ellis, and the description of its general structure given by La- 

 marck,* after an interval of seventy years, is identically the 



* Anim. s. Vert. ii. 10. Bosc, Vers, ii. 216— Lamouroux in 1810 and 1812 

 bad indeed asserted that the polypes with polypidoms coidd not, hi relation to 

 their structure, be compared mth the fresh-water hydra, but that they approxi- 



