PORIFERA 113 



communicate with the paragaster, and flows outwards through the 

 osculum. A third grade is found in sponges such as the calcareous 

 sponge Leucandra (Fig. 100), where the wall of the paragaster is folded 

 a second time, so that the flagellated chambers, instead of opening 

 direct into the paragaster, communicate with it by exhalant (or efferent) 

 canals lined with pinacocytes. 



The three grades of sponge structure (Fig . i o i ) , in which successively 

 the choanocytes line the whole paragaster, are restricted to flagellated 

 chambers, or are still further removed by the presence of exhalant 

 canals, are known as the *' Ascon", '' Sycon'*, and *' Leucon" grades. 

 In many of the sponges whose canal systems are of the third grade, 

 the flagellated chambers are no longer thimble-shaped, but small and 

 round. As the canal system has grown more intricate, complication 

 has taken place also in the skeletogenous layer. It has grown thicker, 

 forming outside the flagellated chambers a layer known as the cortex^ 

 in which the inhalant canals ramify ; and there appear in it branched 

 connective tissue cells which can change their shape. 



The sponges which we have so far considered have skeletons 

 composed solely of calcareous spicules, and their choanocytes are 

 relatively large. They constitute a comparatively small group, the 

 class Calcarea. The majority of the phylum are without calcareous 

 spicules and have relatively small choanocytes. They have usually 

 siliceous spicules, of which there exist many diff"erent types (Fig. 102), 

 characteristic of various groups of sponges, while minor differences 

 distinguish those of the species, which are often only separable by 

 this means. A horny substance, spongin, may occur as a cement 

 uniting spicules, as fibres in which spicules are imbedded, or as a 

 fibrous skeleton from which spicules are absent. The sponges in 

 which the skeleton is in the latter condition constitute the horny 

 sponges (Keratosa), of which the bath sponge (Euspongia, Fig. 103) 

 is an example. Foreign bodies (sand grains, etc.) are often imbedded 

 in the spongin fibres. In a few cases (Myxospongiae) there is no 

 skeleton. The choanocytes of non-calcareous sponges are always 

 restricted to flagellated chambers. Almost without exception these 

 are arranged as in calcareous sponges of the Leucon type, and in 

 most cases the system is made still more intricate by ramiflcations of 

 the paragaster, the irregular appearance of numerous oscula, which 

 put it into communication with the water at many points, and the 

 appearance of "subdermal cavities" and other complications in the 

 outer part of the body. 



The non-calcareous sponges fall into two very distinct classes — 

 the Hexactinelltda, in which there is always a siliceous skeleton of six- 

 rayed spicules (Fig. 102 /), the jelly is absent, and the flagellated 

 chambers are thimble-shaped, as in the simpler Sycons; and the 



