METAZOA PORIFERA. 85 



of the horny sponges is so similar to that of the silicious 

 sponges that a precise description would be mere repeti- 

 tion. According to this author the horny sponges are 

 more nearly related to the silicious than are the silicious 

 sponges among themselves ; so that a separation of an 

 independent order of fibrous sponges does not seem justi- 

 fied from a morphological point of view but only as a 

 matter of convenience. 



One of the simplest sponges belonging to this group is 

 Ammolynthus prototypus Hkl. (PI. 88, fig. i). A cross 

 section (partly diagrammatic) is seen in fig. 2 which 

 exhibits the egg with its nucleus and nucleolus. The 

 sponge that grows from this egg consists of a simple tube 

 with one large opening (fig. i). The body cavity is sim- 

 ple (fig. 2) and without branches, the canal system being 

 similar to that of the Ascones among the Calcarea. The 

 walls of the tubular body are pierced by many pores 

 through which the water enters ; this flows into the large 

 central cavity which is lined with endodermal flagellate 

 cells. No skeleton is developed, but the animal takes up 

 Radiolarian shells (see figs, i, 2) and in this way makes 

 a false skeleton. 



This sponge and the other species of the group to 

 which it belongs are remarkable examples of symbiosis 

 already seen among the Protozoa (see p. 44) . In place 

 of the horny fibers of other keratose sponges it has the 

 tubes of a hydroid which serve the purpose of a support- 

 ing framework. 



Ammosolenia (PL 88, fig. 3) is similar in structure to 

 Ammolynthus but is a colonial sponge corresponding to 

 the Leucosolenia in the calcareous group. 



An extremely interesting form is represented by PI. 89, 

 fig. i. Here in Darwinella australiensis Carter, we 

 have a sponge with spicules made of spongin instead of 

 carbonate of lime or silica. No spicules are wholly min- 

 eral, however, and this being the case, it is not difficult to 

 understand, as pointed out by R. von Lendenfeld, how 



