SILICISPONGI/E 



205 



glass sponges) — have three crossed axes (six threads radiating from a common 

 point) — hence Hexactinellida;. The mesoderm is scanty and in consequence 

 the canals are loose-meshed, lacunar spaces and the ampullae large and barrel- 

 formed. In the Tetraxonia the mesoderm is usually abundant and the canal 

 system well developed. The four-axial spicules of the Tetractinellidae must be 

 regarded as the fundamental skeletal type. From this are derived the compact 

 frameworks of the Lithislids and the monaxial spicules of the Monactinellid;e. 



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

 an extensive framework; or the union is affected 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. HexactinelliDjE, chiefly deep seas; 

 Euplecidla aspergUlum, Venus' fiower-basket. Hyalonema. Sub Order II. 

 TETRAXONIA. Typical are the largely extinct Lithistid^ (some genera — 

 Discodennia — persist in deep seas) and Tehl^ctinellid^: Geodia* Near 

 here apparently belongs OscarcUa* without a skeleton (Myxospongia). Mon- 

 ACTiNELLiD.i;, spiculcs united by spongin (Cornacuspongia); can even be 

 entirely replaced by that substance. Numerous marine forms, and the fresh- 

 water Spongillid.e (Spongilla* Epkydalia*), usually colored green by algae. 

 They are distinguished by formation of gemmulce or statohlasts. At times the 





Fig. 167. — Skeletal structures of sponges (after Schulze and Maas). i, Horn 

 fibre of bath sponge with spongioblasts; 2-7, spicules of, 2, Esperia; 3, 4, Corticum; 5, 

 Mysilla; 6, Telhya; 7, Farrea. 



protoplasm divides into round bodies, as large as the head of a pin and these 

 become surrounded by a firm membrane often strengthened by collar-button-like 

 spicules, the amphidiscs. These statoblasts survive times of freezing or drought. 

 On return of good conditions the contents escape and form small SpongUhc, often 

 utilizing the old skeleton. The spicules entirely disappear and nothing but the 

 spongin fibres remain in the horny sponges, Ceraospongle. The skeleton 

 consists of an organic substance, spongin, which differs chemically from true 

 horn — keratin. This spongin is laid down by peculiar cells, the spongioblasts 

 (fig. 167, i), and it always consists of concentric layers. The fibres interlace, 

 branch, and unite. Best known are the bath sponges; Euspongia officinalis * 

 occurring in the Mediterranean, West Indies, Florida, and other seas in many 

 varieties. Best are the Levant sponges (var. vwUissima). Sponges of com- 

 merce consist only of the skeleton, the animal parts being washed away. Less 

 valuable are Euspongia zimocca and Hippospangia equina* the horse-sponge. 



