Structure and Dcvdopment of a. Horay Sponge. 65 



Beneath the epidei'mis a very lai'ge i^roportion of the 

 eetosonie is occupied by the sand grains above mentioned, 

 but suiTounding these is a considerable quantity of 

 mesodermal tissue. This is for the most part made up of 

 cystench^'me, but stellate mesoderm cells are also present. 



The name CysteiichyiiLe has been applied by Sollas to a 

 peculiar form of tissue not uncommonly met with in the 

 ectosome of sponges. In Stelosjwnrjii.^, the cystenchyme cells 

 are oval or subglobular in shape, measuring about 024 mm. 

 in diameter, and tlie nucleus is small and granular. The 

 protoplasmic strands which connect the nucleus to the cell 

 wall are best seen in unstained preparations mounted in 

 glycerine. They are then seen to form a network, branching 

 and anastomosing inter se. In balsam preparations, owing 

 doubtless to the greater transparency, the i)rotoplasmic 

 strands are not nearly so distinct. 



I may mention here that cystench^'me occurs also in the 

 choanosome of Stelospon<ju.i, but to this point I sliall return 

 later on. 



The stellate mesoderm cells of the ectosome a|)pear to be 

 thoroughly typical. They may be seen investing the grains 

 of sand in a kind of delicate network, the individual cells 

 being mutually connected by long delicate processes. The 

 bod}" of the cell is somewhat granular, and the nucleus is 

 oval and of moderate size. 



(b.) The Choanosome. 



(1) The walls of the inhalant and exhalant canal system. — 

 It will be convenient to give these the first consideration 

 because of their close relationship to the ectosome, from 

 which they cannot be sharply separated. 



The larger or ]n"oximal portions of the inhalant canal 

 system are provided with special walls of mesodermal tissue. 

 The true nature of this lining membrane is difficult to 

 determine. In sections it is seen to consist of a very much 

 vacuolated, gelatinous substance, composed more or less of 

 cystenchyme, but in })arts becoming fibrous. 



The ultimate inhalant lacuna^ have no special mesodermal 

 walls, but here the nuclei of a lining e[)itlielium can be 

 easily detected in thin sections. 



The ultimate exhalant lacuna? also have no special 

 mesodermal walls, but are lined by a delicate flattened 

 epithelium. The larger exhalant canals, or oscular tubes, 

 are, however, provided with most distinct walls, which can 



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