PORIFERA 



123 



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

 rayed spicules (Fig. 103/), the jelly is absent, and the flagellated 

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

 Demospongtae, in which the skeleton, if present, does not contain six- 

 rayed spicules of silica, jelly is present, and the flagellated chambers 

 are almost invariably small and rounded (Fig. 106 C). 



ff.c. 



Fig. 104. A diagram of the structure of a bath sponge (Euspongia). From 

 Borradaile. exh.c. exhalant canal; inh.c. inhalant canal; fix. flagellated 

 chamber; osc. osculum; ost. ostia; sd.c. subdermal cavity; sk. one of the 

 principal pillars of the skeleton, containing imbedded sand grains; sk.' minor 

 fibres of the skeleton. 



Sponges have free larvae, of several different kinds, but all covered, 

 wholly or in part, with flagellate cells, by which they swim. The re- 

 markable feature of the metamorphosfes by which these larvae become 

 the fixed adults is that the flagellated cells pass into the interior, 

 develop collars, and become the choanocytes (Fig. 106). 



Asexual reproduction is found throughout the group. It takes 

 place by the outgrowth and separation of external buds, or by the 

 formation of internal buds or gemmules, enclosed in stout coats. In 



