204 



PORIFERA 



uniting in trunks and these in turn branching to go to the ampullae. 

 The excurrent canals also show a similar tree-like arrangement. Not 

 infrequently extensive subdermal or subcloacal spaces occur. The 

 relations may be more complicated by the development of several 



cloacae, or by the branching of the sponge (fig. 

 164), while still further the branches may 

 anastomose (fig. 165), giving rise to a netwoik. 

 Sponges may reproduce asexually, small 

 portions separating as buds and producing new 

 animals (fig. 88). Usually sexual reproduction 

 prevails. The eggs, which like the spermatozoa 

 arise from mesoderm cells (fig. 160), undergo 

 segmentation and leave the parent as flagellate 

 larvae (fig. 166, A). At fixation a kind of gas- 

 trulation takes place, the blastopore (B) closes, 

 and the osculum, an entirely new formation, 

 arises at the opposite pole. 



FIG. 166. Development 

 of Sycandra raphainis (after 

 Schulze). .4 blastula; B, 

 gastrula at the moment of 

 fixation; ek, ectomesoderm; 

 en, entoderm. 



The sponges are frequently regarded as Coe- 

 lenterata, but scarcely a single homology can l;e 

 drawn between the two. The ccelenterate mouth 

 is different from either pores or oscula. Indeed, 't 

 is disputed whether the collared cells are entoderm. 

 Most sponges possess a skeleton secreted by special 

 mesoderm cells, and this skeleton affords the 

 means, according as it is composed of calcic car- 

 bonate or of silica, of dividing the sponges into two classes. Besides, there are 

 two groups, Ceraospongiae and Myxospongiae, in which the skeleton is respec- 

 tively of horny substance (spongin) or is lacking entirely. These seem to be 

 descendants of the silicious forms. 



Order I. Calcispongiae. 



The calc sponges are exclusively marine and mostly live in shallow water. 

 They are grayish or white in color, of small size, rarely exceeding an inch in 

 length. The skeletal spicules usually project through the epithelium, forming 

 silky crowns in the neighborhood of the osculum. One-, three-, and four-rayed 

 spicules are recognized, these ground forms presenting by unequal development 

 a great variety of shapes. 



Sub Order I. ASCONES. Thin porose walls and central 'stomach.' 

 Leucosolenia* Sub Order II. SYCONES. Cloaca present surrounded by 

 ampulL'e radially arranged. Grantia,* Sycon,* Sycandra * Sub Order III. 

 LEUCONES. A complicated system of branching canals in thick walls 

 connects the ampullae with outer surface and cloacal cavity. Leucetta, Leucortis. 



Order II. Silicispongiae. 



The siliceous sponges are richest in species and occur at all depths of the 

 sea, being frequently noticeable from their size and bright colors. They are 

 subdivided into Triaxonia and Tetraxonia. In the Triaxonia the spicules 

 composing the skeleton appearing as if of spun glass (hence Hyalospongia, or 



