PORIFERA. 



75 



into processes which join similar processes of neighbouring cells and 

 the mesodermal network. The collared cells are, as a rule, confined 

 to special parts of the canal system called the flagellated or ciliated 

 chambers. Their main function is, no doubt, to cause the curreT>t of 

 water which is continually flowing through the sponge. Ciliated 

 epithelium is not found in the Porifera, though in some sponges 

 (Oscar ella lolularis, Plaldna monoloplia, etc.) the ectoderm cells 

 carry flagella. 



We may now proceed to describe in greater 

 detail the various parts of the sponge-body. 



The simplest form of sponge we do not say 

 the most primitive, though it may be so is 

 presented by the Ascon type of the order 

 Calcarea. 



The Ascon-person, which is characteristic of 

 the genus Leucosolenia* consists of a cup- or 

 vase-shaped animal attached by one end, and 

 presenting at the other an opening the oscu- 

 lum. The walls are thin, and consist of ectoderm 

 outside, flagellated endoderm inside, lining the 

 cavity of the cup, and thin mesoderm, containing 

 triradiate calcareous spicules, between the two. 

 They are further pierced by numerous pores, 

 the prosopyles. 



In the Sycon-person (Fig. 65), which is 

 characteristic of the Heterocosla, there is a tube 

 or cup open by the osculum at one end and 

 attached at the other. This tube is lined by 

 flat cells, and gives off all around and through- 

 out its length numerous short diverticula lined 

 by flagellated cells. These are the radial flagellated chambers: 

 they possess, in addition to the one main opening into the cavity of 

 the central tube, which we may call the gastric cavity, numerous 

 minute pores the prosopyles through which water passes from the 

 exterior into the flagellated chambers. These radial tubes, in short, 

 resemble an Ascon in structure, except for the absence of an osculum 

 at their free end. In the simplest Heteroccela the radial chambers 

 stand out freely from the central tube, and do not touch at any 



* This genus comprises Ascetta primordialis and all Haeckel's Ascons. The 

 Olynthus is a hypothetical animal imagined by Haeckel, and closely approaching 

 the Ascon-person in form. 



FIG. 65. Longitudinal 

 section through Sycon 

 raphanus, slightly mag- 

 nified. osculum with 

 collar of spicules; Rt 

 radial tubes which open 

 into the central cavity. 



