144 



PROTOZOA. 



Fig. 39. Section of Astylospov yia 

 prcemorsa, a siliceous Silurian Sponge. 

 (After Roemer.) 



forms indicate that the deposits in which they occur were 

 laid down in a considerable depth of water. 



Of the Silurian Sponges, the only forms which have been 

 definitely shown to be Hexactinellids are Astylospongia (fig. 

 39), Palceomanon, and ProtacJiilleum. In the first of these, 



we have small, more or less 

 globular, unattached sponges, 

 furnished with a cup-shaped or 

 funnel-shaped depression at the 

 summit. The aquiferous canals 

 are partly radial and directed 

 from the surface towards the 

 centre, and partly radial but 

 vertically disposed parallel to 

 the surface, so as to open in 

 the summit-cup. The skeleton 

 consists of hexradiate spicules, 

 which are soldered together so 

 as to form a continuous net- 

 work, the points of intersection of the component rays of 

 each spicule (the so-called " crossing-nodes " ) being solid. 

 Palceomanon is basin-shaped, and with larger lateral canals, 

 but it is hardly generically separable from Astylospongia, 

 and both occur in the Upper Silurian, the latter being also 

 found in the Lower Silurian. In Protachilleum, the sponge- 

 body is mushroom-shaped and stalked, and there is no sum- 

 mit-cavity. 



Very little, indeed, is as yet known as to Devonian Hexac- 

 tinellids the supposed Devonian Sponge described by McCoy 

 under the name of Steganodictyum having been shown to be 

 really the buckler of a Pteraspidian fish. Our knowledge of 

 the Carboniferous representatives of this group is also singu- 

 larly defective ; but we have the extremely interesting fact 

 that the deposits of this age contain, in no inconsiderable 

 numbers, the remains of ancient members of the now 

 living genus Hyalonema. In this genus, comprising the so- 

 called " Glass-rope Zoophytes," there is a comparatively small 

 sponge-body, rooted to the mud of the sea-bottom by a longer 

 or shorter rope of delicate siliceous fibres. In addition to 



