CALCISPONGI^. 225 



a kind of gastrulation takes place, the blastopore (fig. 169, B] 

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

 opposite pole. 



FIG. 168. FIG. 169. 



FIG. 168. Leucetta sagittate*. (After Haeckel.) 



FIG. 169. Development of Sycandra raphanus. (After Schulze.) A, blastula; B, gas 

 trula at the moment of fixation; ek, ectomesoderm; en, entoderm. 



The sponges are frequently regarded as Ccelenterata, but scarcely a 

 single homology can be drawn between the two. The ccelenterate 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 means, according 

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

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

 Myxospongiae, in which the skeleton is respectively of horny substance or 

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

 the silicious forms. 



Order I. Calcispongise. 



The calc sponges are exclusively marine and mostly live in shal- 

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

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

 which arise in the mesoderm usually project through the epithelium 

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

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

 ground forms presenting by unequal development a great variety 

 of shapes (fig. 170). 



Sub Order I. ASCONES. Sponges with thin porose walls and a cen- 

 tral 'stomach' (figs. 161, 167). Leucosolenia* 



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

 radially arranged (fig. 163). Grantia,* Syeon,* Sycandra* 



