Haivey B. Roll — On Fo^nl Sponges. 73 



nmles are often concerned in the prodr.cticn of one large individut.l ; 

 and thi.s fact may j^robabl}^ account for the comparatively very few 

 small sponges that are found." Thu.s, the form of the sponge may be 

 modified, in some instances, by the number of gemmules or ova that 

 may happen to be grouped together, for it is well known that sponges 

 of the same species readily unite when in contact. 



It is necassary to bjar th353 facl;? in view, for i!i mo5t of the higher 

 groups of life, whether living or extinct, variation in form is restricted 

 to within very narrow limits, and therefore it is one of the mast im- 

 portant chai'acters we possess in the determination of the species. 

 Nevertheless, even in the sponges it is not without a value, for in cer- 

 tain fossil genera the more matured individuals appear to be tolerably 

 constant in this respect, as for instance the VentriculUes, hchadites, 

 GuettarcUa, etc., but at the same time the young condition of these 

 genera are unknown to us, or, if so, have been regarded probably as 

 altogether distinct; the youngest individuals which ai-e recognized 

 with certainty have already attained, comparatively speaking, consid- 

 erable 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 

 only a part of the way, as in Siphonia. It-is distinguished from the 

 oscules by its lai-ger 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 some- 

 times 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 oi' SipJwiiia pi/rlformis, 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 intervening 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 maturity. 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. Tlie Oscules. — The oscules are the orifices of the incurrent and ex- 

 current canals. By all authors who have written on the fossil sponges, 

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



