Dr. H. B. mil— On Fossil Sponges. 345 



viduals which are recognizable with certainty having already at- 

 tained, comparatively speaking, considerable dimensions. 



2. The Cloaca. — The cloaca or tubule may be either isolated or 

 grouped. It may extend nearly the entire length of the sponge or 

 onl}^ a part of the way, as in Siphonia. It is distinguished from the 

 oscules by its larger size, the evenness of its walls and often by 

 the orifices of the excurrent canals, or oscules, opening into it. The 

 cloaca is essentially an ejaculatory passage, and in those fistulous 

 sponges having oscules on the outer surface, these latter are the 

 orifices of incurrent, not of excurrent canals, and in the living sponges 

 are sometimes protected by a diaphragm formed of long simple 

 slightly curved (acerate) spicula, but which would necessarily be 

 lost in the fossil. 



In the young sponge the cloaca is sometimes absent, as is often 

 the case in the earlier period in the growth of Siphonia pyriformis, 

 in which the place of the cloaca is occasionally found occupied by a 

 group of small tubules, which ultimately either becomes converted 

 into one large fistulous opening by the breaking down of the inter- 

 vening tissue, or is surmounted by the true tubule. In its earlier 

 condition, therefore, it presents the characters of Jerea, and only 

 becomes converted into a veritable Siphonia as it approaches ma- 

 turity.^ On the other hand, in old fistulous sponges the margins of 

 the cloaca sometimes break down and become fissured, and at length 

 converted into an irregular cavity, in which it is difficult to recognize 

 the characters of the original tubule. 



3. The Oscules. — The oscules are the orifices of the incurrent and 

 excurrent canals. By all authors who have written on the fossil 

 sponges, however, they have been regarded solely in the light of 

 ejaculatory openings, but the study of the recent species has enabled 

 Dr. Bowerbank to ascertain that this is not the case, and that in the 

 tubular and cyathiform sponges, those only which open into the 

 cavities appertain to efferent canals ; while those situated on the 

 exterior lead into the canals which are destined to give passage to 

 the incurrent streams of nutritive fluid. In all the tubular and cup- 

 shaped sponges, therefore, their office may be inferred by their 

 position. In the amorphous sponges they are scattered over the 

 surface either singly or in groups, sometimes on mammillary eleva- 

 tions or ridges, sometimes in pits or depressions, and are probably 

 ejaculatory orifices, imbibition taking place through the pores and 

 interstices of the sponge skeleton. They are permanent organs, and 

 vary greatly in size, proximity, and regularity in their distribution. 

 Occasionally several grooves radiate from the margin of the oscule, 

 and in other species there is no distinct orifice, but the grooves ter- 

 minate internally in three or four small pores, which then supply the 

 place of the single oscule ; but even these are sometimes scarcely 

 perceptible, as in the Silurian Stromatopora (Stellispongia) constellata 

 (Hall). These stellate grooves or "Sillons" are not, however, 

 physiologically distinct from the oscules ; and the smaller grouped 



^ The J erea pyriformis (Lamouroux) is perhaps a distinct species. 



