NORTH AMERICAN PORIFER^. PART II. 483 



tular forms can be said to be tubulai', since the walls are always of such excessive thickness. 

 Ceratella is the extreme aberrant form which represents the arborescent Janthellte among 

 the A2ilysina3. These two representative forms respectively represent two distinct groups 

 of corals, the Janthella, as previously described, being closely similar in the aspect and 

 structure of the skeleton to the genus Antipathes, while the Cei'atelliB approximate in 

 aspect more closely to the Gorgoniacae, though the absence of the large internal fibres con- 

 fines this analogy to the external aspect of the flattened branches and the appearance of 

 the scattered oscules, which look like the dried polyparies of the Gorgonia-stock. 



It is usual to regai'd the commercial Sponges as having no specifically distinct forms, and 

 such is the case when they are compared with the more constant forms of the higher ani- 

 mals, and even of most of the lower classes of the invertebrates. But the extensive collec- 

 tions which have passed under my hands enable me to give a formula which expresses the 

 possible range of variation in every species. The basis of the form is the same as that 

 pointed out by Hseckel and others for the Calcareous sponges ; namely, the simple fistular or 

 vasiform, a single tube, with most of the excurrent oscules on the inner surface. There 

 may be every possible combination of this form, the tubes being either in the same plane, 

 and more or less isolated in a level mass of more solid tissue, or they may be closely approx- 

 imated, forming a solid head, or they may arise into separate branches, each isolated branch 

 being a distinct tube, or association of tubes. Another series of forms have solid branches, 

 in which the oscules owe their existence in the mass to the conjunction of a number of 

 the excurrent canals, and are scattered all over the external surface, as in Chalina. 



This is not the way in which the fistular forms arise, although they have been generally 

 supposed to be identical with the separate cloacal trunks of the solid masses. Thus the 

 tubes may be divided into two classes, those which are continuations of the original cloacal 

 trunks of the young, as in the true Spongite, and correspond to Ha^ckel's idea of a fistular 

 form; and the ordinary fistular form, of which Hlrcinia campana affords admirable illustra- 

 tions. When the latter is very perfect it is perhaps impossible to decide from adult speci- 

 mens alone to which class it belongs, but an examination of the varieties will generally 

 show how it arises, especially if the earlier stages of growth of the colony are investigated. 



The ordinary fistular form is a tube which owes its existence to the growth of the general 

 surface of a previously solid mass, and receives the discharge of the excurrent oscules, some- 

 times these oscules being almost wholly confined to its internal surface, while in other speci- 

 mens they may be scattered equally over this and the external surfiice also. These are the 

 true horaotypes of the primitive cloacal orifice of the embryo, and thus, although the tube of 

 the true fistular or, when widely open, the vase-shaped form, represents functionally a single 

 excurrent trunk and orifice, it is to be distinguished from those which arise directly out of 

 the mass of the sponge, in somewhat the same way that we can distinguish the general 

 cloacal cavity of Pyrosoma from the anal outlets of the individual ascidiaus. It is really a 

 pouch or coecum, form&d by the general surface of the sponge, and is not a continuation of 

 any single internal channel or excurrent tulje, as are the true excurrent oscula.-" Thus we 

 can divide the series of fistular forms very decidedly from the more solid branching forms, 

 though the latter are often found combined with the former on the same colony. From 



' Barrois' observations show that the true canals and the emloJerm in the young sponge after it becomes attached, 

 first grand cloacal trunk arise by a hollowing out of the 



