SPONGES 



13 



cylindrical, and narrower at the base than at the summit, as in tlie 

 case of Tentorium (Fig. 31), and any form of massive sponge may 

 be further complicated by the formation of lobes and irregularities 

 on the surface, or in other ways. In the fossil Siplwnia the 

 massive sponge has developed a stalk, and has the form somewhat 

 of a rose-bud, at the apex of which the relatively small and reduced 

 gastral cavity opens by the osculum (Fig. 11, A and B). 



Two remarkable sponge forms are seen in the genera Tri- 

 hrachion and THs^tirinqa amongst Tetractinellids. Both of them are 



Fig. 17. 

 Asconema setubalense, Kent. (After Wyville Tlionipson.) J. 



to be regarded as massive forms in which the more or less globular 

 body is not fixed, but lies loosely in the mud at the bottom of the 

 sea, and which have developed peculiarities of structure correlated 

 ^vith their mode of life. Thus Trihradiion (Fig. 25) has developed 

 an oscular tube of great length, while in Disi/ringa (Fig. 26) not 

 only is the exhalant ajjcrture prolonged in like manner into an 

 elongated tube, but also an inhalant tube is developed, terminating 

 in a single aperture by which is taken in all the water which 

 enters the canal system. The cavity of the inhalant tube forms a 

 sort of atrial chamber, as it were, in which all the pores are collected, 

 and no pores are found on the surface of the body. Bisi/miga is 



