226 PORIFERA. 



Sub Order III. LEUCONES. A complicated system of branching in- 

 current and excurrent canals in the thick walls connects the ampulla 

 with the outer surface and the cloacal cavity (figs. 164, 168). Leucetta, 

 Leucortis. 



FIG. 170. Sponge spicules. (From Lang.) 



Order II. Silicispongise. 



The siliceous sponges are richest in species and occur at all 

 depths of the sea, being frequently noticeable from their size (up 

 to a yard) and their bright colors. They are subdivided into 

 Triaxonia and Tetraxonia. In the Triaxonia the spicules com- 

 posing the skeleton appearing as if of spun glass (hence 

 Hyalospongia, or glass sponges) have three crossed axes (six 

 threads radiating from a common point) hence Hexactinellidae. 

 The mesoderm. is scanty and in consequence the afferent and 

 efferent canals are loose-meshed lacunar spaces and the ampullaB 

 large- and barrel-formed. In the Tetraxonia, on the other hand, 

 the mesoderm is usually abundant and the canal system well 

 developed. The four-axial spicules of the Tetractinellidse must 

 be regarded as the fundamental skeletal type. From this are 

 derived the compact agglutinated frameworks of the Lithistidae 

 and the monaxial spicules of the Monactinellidae. 



In both groups the spicules may be united by secondary deposits of 

 silica to an extensive framework ; or the union is effected by spongin, 

 which, if the spicules disappear, forms the whole skeleton (horny 

 sponges), or, as in slime-sponges, the whole skeleton may be lost. 



Sub Order I. TRIAXONIA. The HEXACTINELLID^: belonging here live 

 chiefly in the deep sea, and for a long time only a few species were known : 

 Euplectella aspergillum, Venus' flower-basket, tubular, consisting of fine 

 spicules. Hyalonema. Apparently the horny sponges Aplysina and 

 Aplysilla, as well as the slime-sponges, Halisarca,* have descended from 

 this group. 



Sub Order II. TETRAXONIA. Typical representatives are the largely 

 extinct LITHISTID.E (of which some genera Discodermia persist in deep 



