No. 3-] DEVELOPMEyr OF MARhXE SPOXGES. 353 



sponge shown in PL XXV, Fig. 5. In the section drawn, three 

 main efferent canals {cf. c.) are shown, each with its osculum 

 (OS.) and its very irregular set of branches {cf. c' ., cf. c" ., etc.), 

 on the walls of which open the flagellated chambers (/". c). 

 The pores on the surface of this sponge {s. f.) lead into wide 

 chambers {s. d. c), the so-called subdermal cavities, from which 

 run the afferent canals {af. c), carrying water to the pores in 

 the walls of the flagellated chambers. The distinction between 

 subdermal cavities and afferent canals is more or less artificial, 

 for the sharpness with which they are marked off from one 

 another varies within wide limits. They are both parts of 

 the same system, the subdermal cavity being merely a main 

 afferent canal, which is especially enlarged in a tangential 

 direction. The water may enter the chambers in some cases 

 directly from the subdermal cavities, but for the most part it is 

 carried to the chambers by the afferent canals, which branch 

 and twist about, following the irregular course of the efferent 

 canals. The mesoderm between the two sets of canals is re- 

 duced to comparatively narrow trabeculae, in which lie the 

 flagellated chambers, arranged in a much folded but still single 

 layer. The spicules which are not shown in the figure are in 

 the mesoderm, either scattered about or united into a mesh- 

 work or a series of bundles. The genital products are also to 

 be found in the mesoderm, scattered about, as a rule, in any 

 part of the body. 



The structure of a horny sponge, such as the sponge of 

 commerce (Euspongia), is essentially similar to that of the 

 hypothetical silicious sponge I have just described. The differ- 

 ences concern especially the skeleton and the precise manner 

 in which the flagellated chambers are connected with the canals. 

 In the horny sponges the silicious spicules give place to a 

 meshwork of horny fibres, which lie in the mesoderm between 

 the canals to which they lend support, and to the course of 

 which their arrangement is adapted. The flagellated chambers 

 in Euspongia, as in many other horny and silicious sponges, do 

 not open directly into spacious efferent canals (as in Fig. 5), 

 but indirectly by means of special canals, one of which runs 

 from each chamber. And so it is with the afferent canals. 



