CALCItiP0]SGI3i. 



225 



a kind of gastrulation takes place, the blastopore (fig. 100, B) 

 closes, and the osculum, an entirely new formation, arises at the 

 opposite pole. 





"^^i\m 





Fig. 168. 



Fig. IBU. 



Fig. lt)K.— Lfure^n sagittata. (After Hapckcl.) 



Fig. IfJM. — Development of si/rmnlnt irqilianiis. (.\fter S<'liul7e ) J, blastula; B, gas- 

 trula. at the nionient of fixation; ek\ ectoniesoderm; e/i, entodernt. 



The sponges are frequently regarded as C'lxdenterata, but scarcely a 

 single homology can bo drawn between the two. The coilenterate mouth 

 is different from either pores or oscula. Indeed it is disputed whether the 

 collared cells are entoderm. Nearly all sponges possess a skeleton secreted 

 by special mesoderm cells, and this skeleton affords the meaus, according 

 as it is composed of calcic carbonate or of silica, of dividing the sponges 

 into two classes. Besides, there are two groups, the Ceraospongise and the 

 Myxospongi;e, in which the skeleton is respectively of liorny substance or 

 .spongin or is lacking entirely. These, however, seem to bo descendants of 

 the silicious forms. 



Order I. Calcispongise. 

 The calc sponges are exclusivel)' marine and mostlj' live in shal- 

 low water. They are grayish or white in color, of small size, 

 rarelv exceeding an inch or so in length. The skeletal spicules 

 which arise in the mesoderm nsually jiroject through the epithelium 

 and form, especially in the neighborhood of the oscvdum, silky 

 crowns. One-, three-, and fonr-rayed spicules are recognized, these 

 ground forms presenting by uncrpial development a great variety 

 of shapes (fig. 170). 



Sub Order I. ASC'ONES. Sponges with tliin porose walls and a cen- 

 tral 'stomach ' (figs. 161, 107). Leucosolenia* 



Sub Order II. SYCONES. A cloaca present surrounded by ampulto 

 radially arranged (tig. 1G3). Gratiliu,* Sycvu* Si/candra/'' 



