54° 



SrONGES. 



sponges the spicules typically possess four axes and four rays, and resemble 

 caltrops. The fleshy sponges, with little or no skeletal structure, form a second 

 group : while a third group includes the monaxonid or uniaxial sponges, with the 

 skeleton typically built up of needle-shaped spicules, with one axis. The horny 

 sponges form a fourth group. 



Four-Rayed Sponges, — Order Tetractinellida. 



In this order the siliceous spicules of the skeleton have four axes and four 

 rays, and typically are shaped like caltrops. The typical form undergoes numerous 



modifications, one of the commonest consisting 

 in the lengthening of the vertical ray, and the 

 bending of the other three rays towards the 

 long shaft, an elegant anchor -shaped spicule 

 resulting. The anchor form may, however, have 

 originated from the branching of a uniaxial or 

 rod-shaped spicule, and not from the alteration 

 of a four-rayed caltrops form. The three prongs 

 of an anchor may point downwards, upwards, or 

 horizontally outwards, and in the last case they 

 are frequently forked. The Lithistida (stony 

 sponges), one of the groups into which the order 

 is divided, are characterised by the presence of 

 peculiar spicules, known as desmas, in which a 

 minute rod or caltrops is surrounded by con- 

 centric layers of silica ; at the margin of the 

 plate or disc thus formed, branched and often 

 tuberculated processes are given off, which usually 

 join or interlock with those of other spicules to 

 form a dense stony skeleton ; but sometimes the 

 desmas are not linked together, and the lithistid 

 sponge is quite soft. In addition to the larger 

 forms of spicules, such as anchors, which form 

 the skeleton, there are minute, coiled, spiral, or 

 stellate spicules scattered in the flesh. A well- 

 developed crust is frequently present ; and in 

 Geodia the crust is composed of solid siliceous 

 globules packed into a layer, beneath which lie 

 the anchors with the prongs next the crust, and 

 the long pointed shafts passing in centripetally. 

 The four - rayed sponges are divided into the 

 groups Choristida and Lithistida. In the former 

 the spicules are loose and separate ; and in the 

 latter desma spicules are present, and usually 

 ;od or interlocked so as to form a stony skeleton. The Choristida frequently form 

 yellowish white, leathery, nodulated cakes, plates, and crusts. The Lithistida, or 



SILICEOUS SPICULES OF FOUR-RAYED ANCHOR 



sponges (magnified 200 diameters). 



