ELISHA MITCHELL SCIENTIFIC SOCIETY. 33 



from very simple to complex types, found in the calcareous 

 sponges,"^' and in the little group of silicious sponges, 

 the Plakinidae, described by Schulze.f A comparison of 

 these forms goes to show that the simplest Ascon sponge 

 (Olynthus) must be regarded as the ancestral type of the 

 group, and that by the continued folding of the wall of 

 this simple form were produced the more complicated 

 sponges. Further, the exceedingly complex silicious and 

 horn\- sponges must be interpreted as colonies in which the 

 limits of the individual can in many cases no longer be 

 recognized. 



The calcareous sponges offer a series of increasingly com- 

 plex forms, which Haeckel divided into Ascons, Sycons 

 and Leucons. HaeckePs views on the relationship of these 

 'forms must in great measure be accepted to-day, though in 

 certain respects, especially as regards the anatomy of the 

 Leucons, later researches (Polejaeff, /. c.) have shown that 

 he was not always in possession of the real facts of the case. 



The simplest calcareous sponges, or i\scons, which serve 

 as the basis for Haeckel' s hypothetical sponge ancestor, the 

 Olynthus, are too familiar to call for any description. The 

 interesting form, Homoderma sycandra (von Lendenfeld) 

 may, however, be mentioned, in which the body is sur- 

 rounded by radial tubes, after the fashion of a Sycandra, 

 but with this difference: The central cavity as well as 

 the radial tubes is lined with collared cells. A figure of 

 this interesting sponge is accessible in Sollas's article on 

 Sponges in the Encyclopedia Brittannica, or in the Zoologi- 

 cal Articles by Lankester, etc., page 40. 



Homoderma bridges the w^ay from the Ascon type to the 

 simplest Sycons, in which the radial tubes are distinct from 

 one another. A surface figure of such a Sycon {Sycetta 



*Haeckel, Kalkspongieu. Polejaeff, Challenger Report on the Calcarea. 

 fF. E. Schulze, Die Placiniden, Zeit. fur Wiss. Zool. Bd. 34. 



