No. 3-] DEVELOPMENT OF MARINE SPONGES. 35 1 



sponge all the tubes are connected together, and the canals 

 lying'' between them (Intercanals, /;/. can. in the figure) 

 are "complicated. Water enters the intercanals through 

 the openings on the surface (surface pores, s. /.), and 

 passes into the radial canals through the numerous chamber 



pores (c. />.). 



The embryology of the Sycons as far as known confirms the 

 belief that they are derived from the Ascons. Thus Sycandra 

 raphanus passes through a distinctly Ascon phase (Schulze 25), 

 the radial tubes appearing later as outgrowths. The actual 

 development of complicated intercanals such as those of Ana- 

 mixilla has never been witnessed, but a comparison of a large 

 number of forms in which the connection between the radial 

 canals varies within wide limits, makes it pretty certain that 

 the intercanals of a form like Anami.xilla are homologous with 

 the simple ectodermic spaces between the radial tubes of 

 Sycetta or Sycandra ciliata. It is exceedingly probable that 

 the actual development of the complicated Sycons will show 

 that the radial tubes are in young stages distinct from one 

 another, and only later become connected together by bridges 

 of tissue in such a way as to form complex intercanals. And 

 so, we must at present regard the intercanals of a form like 

 Anamixilla as lined with ectoderm. 



Coming now to the Leucons, we find that Polejaeff's descrip- 

 tion of the anatomy of this family accords with their derivation 

 from the Sycons quite as well as did Haeckel's more imagina- 

 tive conception of the structure of these forms. Taking one 

 of the simplest of Polejaeff's types, let us compare it with a 

 Sycon. In PI. XXV, Fig. 2, is shown part of a transverse 

 section of Leucilla connexiva. Such a form is obviously 

 derived from a Sycon by the evagination of the wall of the 

 paragastric cavity at certain points (.r,.r). These evaginations 

 give rise to numerous diverticula of the central cavity, which 

 constitute efferent canals, cf.c. The radial chambers are at 

 the same time thrown into groups, each group openmg mto 

 one of the new diverticula. The intercanals {In. can.) pene- 

 trate as before between the several radial chambers, bringing 

 water to- the chamber pores (c.p), the complexity of their 



