90 Dr. C. Collingwood on Recurrent Animal Form, 
have certain compound forms made up of numerous mem- 
branous or calcareous cells, upon a common axis or stem, which 
branches in a plant-like manner, each cell being the habitation 
of a distinct animal. These are their homomorphie characters ; 
now let me state what are the special characters of each group. 
First, Hydroid Polypes: mouth with filiform, simple tentacula ; 
stomach excavated in the cellular substance of the body; no 
distinct muscular apparatus; body contractile in all its parts, 
gemmiparous externally. Secondly, Polyzoa: bodynot contractile, 
symmetrical ; mouth and anus separate ; gemmiparous and ovi- 
parous. It therefore appears that the Polyzoa are minute Mol- 
luses, differing in all their homologies from Polypes. Let us 
next inquire of which group the Polyzoary form is typical. 
Clearly not of the Mollusca, which are for the most part of very 
different form ; and equally clearly it is typical of the Polypes, in 
which Class it assists their analogy with vegetable forms. The 
Polyzoary form, then, is aberrant from the Molluscan, and 
typical of the Hydroid Polypes. Why this form is best adapted for 
the life of Polypes I am not required to prove, but only why (that 
being granted) it is also the best form for the Polyzoa. Next, 
Jet us inquire what differences exist in the form of the animals 
themselves. In the Polype there is a gelatinous substance hol- 
lowed out into a stomach, a single aperture serving the purposes 
of taking in food, and passing out rejectamenta and ova, this 
common outlet being surrounded with a circlet of gelatinous 
contractile tentacles, armed with nettling capsules. But the 
Molluscoid has an cesophagus, stomach, gizzard, intestine, di- 
stinct anus, besides a liver and nervous system. In none of 
these particulars has it any relationship with Polypes; but the 
mouth is surrounded with a circlet of tentacles, not indeed like 
those of Polypes, simple and contractile, but uncontractile, and 
covered with vibratile cilia. They are probably the homologues of 
the labial palpi of other Molluses. This circlet of tentacles then 
is the great point of resemblance between Molluscoids and Polypes 
—in the latter the common arrangement, in the former arising, 
as it were, from an accident or variety of organization; and yet 
is it not easy to perceive that the common possession of ten- 
tacles exhibited by Polypes and Polyzoa implies a very great 
similarity, nay, almost identity, in one of the most important 
of habits, namely the mode of procuring food ? 
Having so far established a eommunity of habit between them, 
let us next refer to the grand organic distinction which is im- 
plied in the widely different form of the digestive apparatus. In 
the Polypes, the rejectamenta being passed out by the mouth, 
such animals are well fitted doubtless for living in cells with a 
single aperture ; the Mollusca, however, have an intestinal canal, 
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