POEIFERA (sponges). 



35 



Euretidse or Craticularidci3 (Jurassic to Eecent) : mostly Gallery 

 cup-shaped or funnel-shaped ; spicular nodes simple and '^iq^i2^ 

 imperforate ; canal openings large, simple, ending blindly in Wall-cases 

 the skeleton. Mellitionidse (Cretaceous, Eocene). Coscino- 7, 8a. 

 poridfe (Cretaceous to Eecent) : in addition to the cup and 

 funnel shape, many have thin walls folded into a series of 

 flanges, e.g. in the Cretaceous Guettardia ; surface canal- 

 openings small, usually arranged in quincunx. Stauro- 

 dermidse (Jurassic and Cretaceous) : usually funnel-shaped 

 or cylindrical, with an irregular skeletal mesh and a 

 definite dermal layer in which are large cross-shaped spicules. 

 Callodictyonidse (Cretaceous). Ventriculitidfe (Jurassic and 

 Cretaceous) : mostly funnel-shaped, with a thin wall thrown 

 into vertical folds which are usually arranged radially 

 (Fig. 10) ; spicular nodes hollow or lantern-shaped (Fig. 9 a) ; Wall-case 

 the base of the sponge has root-like extensions of spicular 

 fibres. Coeloptychidee (Cretaceous) : mushroom-shaped with 

 a thin wall thrown into radial folds and enclosed in a 

 perforate dermal layer ; canal-openings in rows on the ridges 

 of the under surface ; lantern nodes. M^eandrospongidas 

 (Cretaceous to Eecent) : pear-shaped, sack-shaped, or nodose 

 masses, with a thin wall thrown into numerous folds, which 

 join one another irregularly and are often partly or wholly 

 enclosed by a fine spicular membrane, e.g. Camerospongia, 

 Cystos]Jongia, and Flocoscyphia. 



Class III.— DEMOSPONGIAE. 



Silicispongiae without triaxon spicules. These are the 

 commonest sponges of the present day, most familiar in the 

 freshwater sponge and the bath-sponge, but found in all 

 waters in the most varied surroundings. Palaeontologists, 

 however, are only concerned with those that retain siliceous 

 spicules. According to the form of those spicules they may 

 be divided, somewhat artificially, into two Sub-Classes. 



Sub-Class I. — TETRACTmELLiDA. Demospongiae 

 typically with tetraxon spicules (Fig. 11). 



Order I. — Choristida (to which the term Tetractinellida 

 is sometimes restricted). Spicules four-rayed and not joined 

 into a rigid network. The simplest spicule has the form 

 of a caltrop (Fig. 11 a, c). In others one ray is elongated, 

 forming a shaft from which the other rays project as three 

 prongs ; this trident shape is called a triaene and is subject 

 to much further modification (Fig. 11 f-k). With these 



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