( CELENTERATA 



55 



take in the food in precisely 

 the same way as do the 

 Choanoflagellata ; the water 

 then passes out through the 

 osculum. It was the pres- 

 ence of these collared, flagel- 

 late cells that led zoologists 

 some years ago to classify the 

 sponges as colonies of flagel- 

 late Protozoa. We now know 

 that this view was incorrect, 

 for there is not only an ecto- 

 derm and an entoderm, but 

 between is a well-developed 

 mesenchyme, in which is 

 formed the skeleton, which 

 is present in almost all 

 sponges. This skeleton is 

 secreted by the mesenchyme- 

 cells and may consist of cal- 

 cium carbonate, or of silica, 

 or of spongin, a substance 

 which is chemically allied to 

 silk, — it forms a fibrous net- 

 work. In some sponges we 

 find both siliceous spicules 

 and spongin fibers. 



There are several methods 

 of reproduction. In many 

 sponges budding occurs, and 

 the individuals thus produced 

 often fuse with one another, 

 forming huge colonies (Fig. 

 39). This fusion may be so 

 extensive that it is impossible 

 to determine the number of 

 individuals in the colony ; in 

 other cases the colony is of 

 such a form that the indi- 



FlG. 38. Sycon gelatinosum. Transverse section 

 through a portion of the body-wall. The arrows 

 indicate the course of the water from the outside 

 to the central cavity or coelenteron. up, apopyle ; 

 dc, dermal cortex ; di, diaphragm ; ec, ectoderm ; 

 em, young embryo ; em', older embryo; en, ento- 

 derm : exc, excurrent passage ; ^e.gastral cortex ; 

 IC, incurrent canal ; ov, ovum ; PC, coelenteron 

 or paragastric cavity; pm, pore membrane; 

 pp. prosopyles; R, radial canal; sp, triradiate 

 spicules; sp' , oxeote spicules; sp", tetraradiate 

 spicules. (After Parker and Haswell.) 



