112 



COMPAEATIVE ANATOMY. 



gastric pores), which have consequently a dermal and gastric orifice, 

 is generally very great ; their number is dependent on that of the 

 spaces which are bounded by the rays of the spicula (cf. Fig. 36, o). 

 These characters are very distinct in the lowest forms of the Calci- 

 spongia?, the Ascones (Olynthus). 



The development of diverticula of the enteric cavity gives rise to 

 a second form; the diverticula are continued into the correspondingly 

 thickened ectoderm, where they form more or less branched canals ; 

 from these, again, fine canals, which are also branched, open into the 

 dermal pores. The enteric cavity, as it becomes more and more 

 divided into branched canals, loses its importance as a stomach, and 

 at the same time its endodermal investment, which is now limited to 

 the branched canals. But the endodermal layer does not extend 

 over all of them, but is finally restricted to their diverticula, which 



are thus converted into 

 the so-called ciliated 

 chambers. Thus the 

 function of the enteric 

 tube passes more and 

 more from its primitive 

 locality into the addi- 

 tional spaces, which 

 are gradually developed 

 from it. 



The subjoined figure 

 (Fig. 42) represents the 

 last stage in which the 

 endoderm invests the 

 ciliated chambers only 

 (w). Modifications of 

 this form which obtains 

 in the group of the Leu- 

 cones, among the Cal- 

 cispongia3, are formed 

 by the union of the 

 branched canals and of 

 the ciliated chambers 

 one with another, 

 whence arise retiform 

 canal systems. The 

 Siliceous and fibrous 

 Sponges conform to 

 this type. 



A third form arises 

 by the formation of 

 closely adjoining canals, 

 directed radially to the 

 stomachal cavity, which in their characters correspond to the simple 

 Ascon form, but which generally communicate with the exterior 



Fig. 42. Diagram of the gastric system of a Leucou 

 (Dyssycus ananas), where branched canals are 

 developed, o Month, g Enteric cavity. ^> Dermal 

 canals. w Ciliated chambers. The difference between 

 the ectoderm and endoderm is represented in the same 

 way as in the previous figure (after E. Hiickel). 



