246 ARTHUR BENDY. 



VI. The Origin and Phylogeny of the Calcarea 

 Heteroccela. 

 No one will probably dispute the now well-established 

 hypothesis that the Calcarea Heteroccela are descended from 

 some one or more ancestral Homoccela. The Homocoela are 

 undoubtedly the more primitive of the two great groups into 

 which Polejaeff (8) divided the calcareous sponges, and the only 

 question which we need discuss is the process by which the 

 Homoccele gave rise to the Heterocoele type. 



In my memoir on the organisation and classification of the 

 Homocoela (1) I proposed to divide the sole genus, Leucoso- 

 lenia, into three sections, according to the nature of the canal 

 system, for even in the simplest of sponges the canal system 

 exhibits a considerable amount of variation. To these sections 

 of the genus I applied the names S implicia. Reticulata, and 

 Radiata. The Simplicia include such simple Olyuthus 

 types as never form colonies, and also those colonial forms in 

 which the whole colony consists of individuals (Ascon-persons) 

 which may branch, but which never form complex anastomoses 

 nor give off radial tubes, so that the individuality of the 

 different members of the colony is always recognisable. In 

 the Reticulata the sponge colony forms a more or less 

 complex network of branching and anastomosing tubes, and it 

 is no longer possible to distinguish the individual Ascon-persons 

 of which the colony is composed. The Radiata include such 

 species as exhibit a radiate structure — the sponge consisting of 

 a single central Ascon-tube, from which other tubes are 

 budded off radially. 



In all species of Leucosolenia it is supposed that the 

 whole of the primitive gastral cavity is lined by a layer of 

 collared cells, and that when, as in my type D of the Reticu- 

 lata, exhalant canals exist which are not lined by collared 

 cells, these canals (pseudogasters) really lie outside the 

 sponge, and are probably formed by upgrowth of the colony 

 around them. 



The Olynthus type^ consisting of a simple sac lined by 



