Dr. J. E. Gray on the Arrangement of Sponges. 285 



ix. p. 446). The sponge olive, formed of a fleshy or 

 horny skeleton, and strengthened by regular siliceous 

 spicules which are secreted by it ; ovisacs membranaceous, 

 scattered in the substance of the sponge. 



Order III. Potamospongia (Ann. & Mag. Nat. Hist. 1872, 

 ix. p. 461). The sponge green ; skeleton* strengthened 

 by regular siliceous spicules ; ovisacs cartilaginous, 

 strengthened by fusiform or birotate siliceous spicules. 

 (See the divisions proposed in the ' Annals ' above quoted, 

 p. 461.) 



Order IV. Calcispongia. The sponge strengthened by 

 regular calcareous, generally three-rayed, spicules. 



Order I. Arenospongia. 



The sponges of this order, which vary in shape from being 

 discoidal, massive, to dendroidal, are peculiar for collecting 

 together the sand or fragments of spicules, which are abundant 

 at the bottom of the sea, for the purpose of giving strength 

 and consistence to their structure ; and these answer the same 

 purpose as the siliceous or calcareous spicules which are 

 secreted by the other marine and freshwater s])onges. The 

 quantity of horny matter covering the sand, and the quantity 

 of sand enclosed by it, are very different in the different species 

 of these sponges. It sometimes forms a thick, fibrous, horny 

 skeleton, with only a single layer of sand in the centre of the 

 fibre ; and in some species this sand is only found in the 

 thicker part of the horny skeleton. In other species the sponge 

 seems entirely formed of sand merely kept together by a thin 

 coat of homy matter. 



In the an-angement I proposed in 1867 I placed the sponges 

 of this group in two families, Dysideido3 and Xenospongiadae, 

 placing the latter family in a subsection which I called sand 

 sponges (Arenospongiadge), and the other family with the 

 Ceratospongia. But more mature consideration has induced 

 me to increase the suborder Arenospongia and put them 

 together ; for it is a very important element in the occonomy of 

 the animal that one family collects together the ready formed 

 siliceous bodies, and the other secretes the siliceous or cal- 

 careous spicula by which the body of the sponge is strength- 

 ened. 



This order consists of two families : — 



1. Xenospongiadae. 



Sponges discoidal, strengthened with irregularly placed sand 

 and fragments of spicules. 



