34 JOURNAL OF THE 



primitiva) is given in \^osmaer. ^^~ In the majority of Sycons, 

 however, the radial tribes are not distinct, but are con- 

 nected together more or less b}' strands of mesoderm cov- 

 ered with ectoderm. The complicated ectodermal spaces 

 thus formed, which lie between the radial tubes, are known 

 as intercanals. Water enters the intercanals through the 

 openings in the surface (surface pores) and passes into the 

 radial canals through the openings in their walls (the primi- 

 tive pores — so-called chamber pores). The embryology of 

 the Sycons, as far as known, confirms the belief that they 

 are derived from the Ascons. Thus Sycandra raphanits 

 passes through a distinctly Ascon phase, the radial tubes 

 appearing later as outgrowths. The actual development 

 of complicated intercanals, such as those just mentioned, 

 has never been witnessed, but the comparison of a large 

 number of forms in which the connection between the 

 radial canals varies w^ithin wide limits makes it pretty cer- 

 tain that they are homologous with the simple ectodermic 

 spaces between the radial tubes of Sycetta. It is exceed- 

 ingly probable that the actual development of the compli- 

 cated 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 so as to form com- 

 plex intercanals. And so we must at present regard the 

 intercanals as lined with ectoderm. 



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

 scription of the anatomy of this family accords with their 

 derivation from the Sycons, quite as well as did Haeckel's 

 more imaginative conception of the structure of these 

 forms. Taking one of the simplest of Polejaeff's types, 

 Lcncilla connexiva (PI. VI, fig. i<t, Polejaefif /. 6\), let us 

 compare it with a Sycon. Such a form is obviously de- 

 rived from a Svcon bv the eva^^i nation of the wall of the 



*Vosniaer, Bronn's Klass. and Ordnvingen, Spongien, Taf. IX. .Schulze, Zeit. fur 

 Wiss. Zool. Bd. 31. 



