SPONGES 31 



To sum up the facts with regard to the structure of the 

 Olynthus, as found in a calcareous sponge, it is seen that its body 

 wall is built up of two distinct layers, and contains five kinds of cells 

 and their products ; namely 



(1) The dermal layer, divided into a more external contractile 

 stratum, the flat epithelium and the porocytes, and a more internal 

 parenchymal or skeletogenous stratum, the spicules and their cells, 

 embedded in a gelatinous ground substance. 



(2) The gastral epithelium, consisting of the collared epi- 

 thelium. 



(3) The wandering cells, which do not constitute a distinct 

 tissue or cell layer, but are found scattered in all parts of the body 

 wall. At certain seasons, some of these cells become germ cells ; 

 hence the wandering cells and the reproductive cells may be in- 

 cluded together under the general term archaeocytes. 



It is possible to imagine, however, a still simpler type of 

 Olynthus than this, one namely in which a skeletogenous layer has 

 not been evolved. The dermal layer would then consist of a single 

 layer of epithelium and of the porocytes. Such an organism 

 would represent the simplest conceivable type of sponge, and might 

 be termed the Protolynthus. A Protolynthus stage is recognisable, 

 as will be seen, in a contracted, pupal form, in the embryonic 

 condition of Ascons, but as a fully developed and functionally 

 active organism it is not known to occur, even as a transitory stage, 

 in the life-history of any existing sponge. 



From the Olynthus as a starting-point we may now consider 

 the organisation of sponges in general. 



(a) Canal System. All the cavities of the body traversed by 

 the currents of water which nourish the sponge, from the time they 

 enter by the pores until they pass out by the osculum, are termed 

 collectively the canal system. In the Olynthus the canal system has 

 been seen in its simplest type. In other forms it may attain to a 

 high degree of complexity, but its general evolution can neverthe- 

 less be reduced to simple processes of growth on the part of the 

 primitive Olynthus (Protolynthus), resulting in a folding of the 

 wall, and accompanied by a restriction of the collar cells to certain 

 regions. In the gradual and continuous process of differentiation 

 three distinct grades or types of organisation can be distinguished 

 which, though connected by numerous transitions, may yet be con- 

 sidered as three styles of architecture, so to speak, under which all 

 existing forms may be classified. 



First Type of Canal System. As an example of this type may be 

 taken the Olynthus itself (Fig. 43), of which the structure has 

 already been described. The parts of the canal system here are 

 pores, gastral cavity, and osculum. 



This type of canal system is only found in Ascons amongst 



