178 PORIFERA. 



siliceous. The oldest known species of Astrceosfio?igia appear in the 

 Silurian, but the genus is principally characteristic of the Devonian 

 system. 



Order 8. Heteractinellid^e. — This group has been founded 

 by Hinde to include certain Palaeozoic siliceous Sponges in which 

 the skeleton-spicules consist of " an indefinite number of rays, vary- 

 ing from six to thirty, radiating from a common centre " (fig. 69). 

 The body-spicules are irregularly disposed and are not fused with 

 one another ; but the dermal spicules are interwoven together, and 

 their rays are partially or completely fused with one another. The 

 genera Tholiasterella and Asteractinella (fig. 69), included by Hinde 

 in this group, are confined to strata of Carboniferous age, and are 

 only known by detached spicules or imperfect fragments. 



Class II. Calcispongi^e. 



The class of the Calcareous Sponges comprises, as the name im- 

 plies, all those sponges in which the skeleton is composed of spicules 

 of carbonate of lime. The spicules vary considerably in form, and 

 different forms are often associated in the same sponge. In living 

 Calcisponges the spicules are never fused with one another, nor 

 united by horny fibre, but in the group of fossil Calcisponges de- 

 scribed by Zittel under the name of Pharetrones, the skeleton is 

 formed by a reticulated calcareous fibre which is " wholly composed 

 of spicules in close approximation to each other, and as closely in- 

 terwoven together as the strands of a rope " (Hinde). The follow- 

 ing are the principal types of spicules found in the calcareous 

 Sponges, (a) Simple uniaxial spicules, which may be associated 

 with the other forms of spicules, or, as in Pharetrosflongia, may be 

 the only forms present, (b) Three-rayed spicules, in which all the 

 rays are in one plane and the rays and angles are equal (figs. 70 and 

 71, b). In some cases the spicules are triradiate, but two of the 

 three rays are paired and equal, and the third ray is longer or 

 shorter than the other two. (c) Four-rayed spicules, with three 

 rays equal and in one plane, but with a fourth ray directed at right 

 angles or obliquely to the others. 



The recent Calcisponges are all of small size and are inhabitants 

 of comparatively shallow water, all being marine. By Vosmaer the 

 Calcispongice are divided into the following two orders : — 



Order i. Homoccela. — In this order there are no special 

 " flagellated chambers," and the canal-system can hardly be said to 

 exist in a definite form ; but the thin walls of the sponge (fig. 70) 

 are perforated by numerous pores opening directly into a general 

 cavity, which is lined by ciliated epithelium and terminates on 

 the surface by a single osculum. The order corresponds with the 



