COMMON SPONGES FOUR-RAYED SPONGES 



3515 



THE COMMON SPONGES Class Demospongia 



The common sponges include all those which do not come under the designation 

 of calcareous or glass-sponges. A negative definition is unsatisfactory, but it is 

 difficult to frame a positive one which will apply to all divisions of this class. Most 

 common sponges are siliceous, while such as are horny are probably derived from 

 siliceous types. They are divided into four orders. In the first or four-rayed 

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

 trops. 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 typic- 

 ally are shaped like caltrops. The typical form un- 

 dergoes numerous modifications, one of the com- 

 monest consisting in the lengthening of the vertical 

 ray, and the bending of the other three rays to- 

 ward 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 downward, upward, or 

 horizontally outward, and in the last case they 

 are frequently forked. The Lithistida (stony 

 sponges) , one of the groups into which the order 

 is divided, are characterized 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 



SILICEOUS SPICULES OF FOUR-RAYED 

 ANCHOR SPONGES 



(Magnified 200 diameters.) 



