148 



PROTOZOA. 



come together at an angle of 120, their extremities being 

 divided into processes by means of which contiguous spicules 

 interlock with one another ; while the fourth arm lies in a 

 different plane and forms a cylindrical shaft from which the 

 other three spring. The spicules are not united by a siliceous 

 cement ; but by the interlocking of their ends the skeleton 

 forms a more or less continuous framework, the meshes of 

 which are more or less irregular and curvilinear. In the 

 recent species " flesh-spicules " are also present. 



The fossil Litliistidce 1 have not yet been so extensively 

 worked out, as is the case with the fossil Hexactinellids ; but 

 they are said by Zittel to be represented in the 

 Silurian (Aidocopium, Calatliium (?), and Eo- 

 spongia (?)). The same observer has likewise 

 indicated their abundant occurrence in the 

 Jurassic period ; but it is not till we reach 

 the Cretaceous period that we are confronted 

 with the maximum development of this group. 

 Of the Cretaceous Lithistids, by far the most 

 important is the familiar and widely distrib- 

 uted genus Siphonia, the structure of which 

 has been investigated by Mr Sollas. In this 

 genus the sponge-body consists of a variably- 

 shaped head, supported upon a longer or 

 shorter stem, and attached thereby to some 

 foreign body (fig. 44), but the stem may be 

 wanting, when the sponge is attached by 

 diverging root -fibres. The exhalant water- 

 canals open at the summit of the sponge, 

 usua iiy by oscula situated within a cup- 



J f 



shaped apical cavity, w T hile the inhalant 

 canals open by " pores " on the lateral sur- 

 faces. The skeleton - spicules (fig. 45, B) are furnished 

 with three diverging arms, are bifurcated, and furnished 



1 Since the above was written, Professor Zittel has published a detailed 

 memoir on the Lithistidce (N"eues Jahrbuch fur Mineralogie, &c., 1878), in 

 which this difficult group is systematically worked out. The general results 

 above stated are, however, not affected by this, though our knowledge of the 

 fossil forms is immensely increased. 



Fig. 44 - 



ficus, a Cretaceous 



sponge. 



