34 PORIFERA 



chambers and open into the gastral cavity. Further 

 complications, such as branching of the canals, may occur. 

 The thick wall of these more complex sponges is formed 

 mainly of the gelatinous layer. 



In a sponge consisting of a single individual, the form 

 depends mainly on the relative rates of growth in different 

 directions, and may be cylindrical, vase-like, globular, 

 discoiclal, etc. In a compound sponge the form depends 

 also on the way in which the young individuals of the 

 colony are attached to the parent, and in addition, on their 

 remaining free or becoming fused together ; in the latter 

 case the individuals of the colony are frequently dis- 

 tinguishable by their oscula only; when the individuals 

 remain free, arborescent or bushy colonies may result ; if 

 they become fused, the sponge may be fan-shaped, funnel- 

 shaped, cup-like, tubular, mushroom-shaped, massive, en- 

 crusting, etc. 



Nearly all sponges are attached to some foreign object 

 — generally by the base of the sponge, but in forms which 

 are fixed in the mud, especially deep-sea forms of the 

 Hexactinellida, and in some Tetractinellida, this fixation is 

 by means of a root-tuft or rope of long spicules. 



In nearly all sponges there is a skeleton, which serves 

 to support the canals and chambers and also for protection. 

 This skeleton may consist of fibres of a horny substance, 

 similar to silk in composition, and known as spongin; or 

 of mineral particles, termed spicules (fig. 8, 6), composed of 

 carbonate of lime or of colloid silica ; or it may consist of 

 both siliceous spicules and spongin. Those forms only 

 which have either a siliceous or calcareous skeleton are 

 definitely known as fossils. Each spicule consists of a 

 number of rays or arms, coming off from a centre, which 

 is the point where the formation of the spicule commenced. 



