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 limit myself in this chapter ; and should the 
reader find the outline 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 c< 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. Bose, Vers, ii. 216. — Lamouroux in 1810 and 1812 
had indeed asserted that the polypes with polypidoms could not, in relation to 
their structure, be compared with the fresh-water hydra, but that they approxi- 
