ii6 



THE INVERTEBRATA 



Demospongiae^ 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. 105 C). 



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

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

 remarkable feature of the metamorphoses by which these larvae be- 

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

 develop collars, and become the choanocytes (Fig. 105). 



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 



Fig. 103. A diagram of the structure of a bath sponge (Euspongia). From 

 Borradaile. exh.c. exhalant canal; inh.c. inhalant canal; fl.c. 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. 



some cases (Spongillidae) the gemmules are remarkable in that they 

 originate as clumps of the amoeboid cells of the parent. They will 

 stand freezing or drought, and carry the species through unfavourable 

 conditions. The power of regeneration and repair is possessed by 

 sponges in a high degree, and they can be propagated artificially by 

 cuttings. 



Sponges are found in all parts and at all depths of the sea. Only 



