No. 3-] DEVELOPMENT OF MARLXE SPONGES. 357 



Plakina trilopha goes a step farther in the direction of com- 

 plexity than does Plakina dilopha. It has probably been 

 derived from the latter species (compare Figs. 9 and 10) 

 by the appearance of secondary folds in the radial efferent 

 tubes ; by a transformation of the basal cavity into a system 

 of lacunae, owing to the increase in number of the connecting 

 strands of tissue between the basal layer and the part of the 

 sponge containing the flagellated chambers ; and by a compli- 

 cation in the afferent canals in consequence of which they do 

 not open each by a single aperture but by a number of small 

 apertures, the surface pores {s. p.). 



Schulze's conclusion that these species all lie in one line of 

 descent, that is that the second has been derived from the 

 first, and the third from the second, receives as much support 

 from a study of the spicules, as of the canal system. But on 

 this head, reference will have to be made to the original paper. 



From comparative anatomy we conclude that the phylogeny 

 of the sponges is something as follows: The Olynthus is the 

 common ancestor of the group. The outgrowth of radial 

 tubes gave rise to the Sycon type. The growth of the meso- 

 derm and development of new endodermic diverticula, coupled 

 with the metamorphosis of radial tubes into flagellated cham- 

 bers, produced the Leucons. The non-calcareous sponges have 

 been derived from types with a canal system more or less like 

 that of the Leucons. And the conclusion with regard to the 

 germ layers is that the efferent system is entirely endodermic, 

 and the afferent system entirely ectodermic. 



Evibryological Evidence. — Let us see now how far the 

 known facts of development support the above conclusions. 

 The evidence from the calcareous sponges (Sycandra passes 

 through Olynthus stage) has already been given. Several of 

 the non-calcareous sponges (Oscarella lobularis, Reniera fili- 

 grana, Chalinula fertilis, Plakina monolopha) run through a 

 stage known as the Rhagon (Sollas), which it is permissible to 

 regard as the ontogenetic representative of the Sycon type. 

 The rhagon of Oscarella (Heider 9) is shown in PI. XXV, 

 Fig. II. Regarding it, as seems best, as equivalent to the 

 Sycon type, it will be noticed that the radial tubes of the 



