4 THE CRAG POLYZOA. 



asunder, or to the whole growth being composed of a similar decomposable organic 

 material. In the former case the minuteness of the separate particles would render them 

 wholly inappreciable ; and as regards the latter it is clear that we can expect to find in the 

 fossil state only those forms of Polyzoa the walls of whose cells contain such an amount of 

 earthy matter as will enable them to maintain their figure when the organic material is 

 removed. And since one or other of the above conditions obtains in a large proportion 

 of the class, it is only of a limited number of families that we can expect to discover the 

 fossil remains in the older sedimentary deposits. 



The general structure and physiological relations of the animals composing this class, 

 notwithstanding the diversified forms exhibited in their skeleton-remains, are extremely 

 simple, and conformable to a very uniform type throughout. 



It has been before said that the Polyzoa are always associated into compound growths, 

 made up of a congeries of individuals, which though distinct yet retain some degree of 

 inter-communication, comparable in kind perhaps, though not in degree, to what obtains 

 in many of the compound Ascidians. That this community exists is proved by the other- 

 wise inexplicable circumstance that the polyzoaria in many instances present elements 

 common to the whole growth, and not belonging specially to any individual. The chief 

 bond of connexion would appear to reside partly in the continuity of the external integu- 

 ment, and partly also, in all probability, in a slow interchange of the vital fluid with 

 which the cavities of the cells are charged. 



Each individual Polyzoon may be briefly described as possessing a saccular body and as 

 having two orifices, an oral and an anal, which, however, are not at opposite ends of the 

 body, but in close approximation. The oral orifice or mouth is surrounded with a single 

 row of non-contractile tentacles, which are hollow, seated upon a thickened band or ring 

 — the lophophore, — and furnished on the exterior with vibratile cilia, by whose action the 

 nutriment is conveyed to the mouth, and a current kept up on the surface of the tentacles 

 for the purpose of respiration. 



The walls of the saccular integument, which may be wholly membranous, or, as is 

 more usual, partly soft and membranous, and partly calcareous and rigid, or fleshy, is 

 attached round the neck, as it may be termed, immediately behind the lophophore, so as 

 to form a completely shut sac, except where it is perforated for the passage of the fajces, 

 but as the Avails are there also continuous with those of the intestine, it results that a 

 closed cavity is formed by them, in which are contained the alimentary and reproductive 

 organs, together wdth the rudimentary nervous system. The upper part of this sac, or 

 that immediately succeeding its attachment, invariably remains flexible and membranous, 

 whilst the remaining portion is usually solidified by the deposition in its tissue of 

 carbonate of lime, or of a horny substance resembling chitine or perhaps conchiolin, in 

 chemical and physical properties. 



The walls of the sac are constituted of two membranes, an internal and an external — 

 the endoderm and ectoderm, or as they have been termed by Professor AUman, the ectocysi 



