76 THE INVERTEBRATA 



Collozoum (Fig. 32 B). As Thalassicolla, but with central capsules 

 united by their extracapsular protoplasm into a colony; and each 

 capsule contains several nuclei. 



Heliosphaera (Fig. 68 A). As Thalassicolla, but the skeleton has 

 the form of a lattice-work on the surface of the body. 



Actinomma (Fig. 68 B). As Heliosphaera^ but the skeleton consists 

 of several lattice spheres, formed successively at the surface as the 

 animal grows, with radial struts joining them. Ultimately the inner- 

 most sphere may lie in the nucleus. 



Acanthometra (Fig. 69 A). (Suborder Acantharia.) A skeleton of 

 radial spicules of strontium sulphate meeting centrally in the central 

 capsule; nuclei numerous; yellow cells intracapsular. Remarkable 



skA sk.2 



^-nu. 



cps. 



Fig. 68. A, Heliosphaera inermisy x 350. After Hertwig. B, The skeleton of 

 Actinomma. After Biitschli. sk. skeleton; cps. central capsule; nu. nucleus. 

 The yellow cells are shown, but not labelled, in A. 



Structures known as " myophrisks ", surrounding the spines of this 

 genus at their junction with the calymma, are contractile and are used 

 in the regulation of the diameter of the body. 



Lithocircus (Fig. 37). (Suborder Nassellaria.) A siliceous skeleton 

 in the form of a ring, bearing spines. Yellow cells extracapsular. 



Aulactinium (Fig. 69 B). (Suborder Phaeodaria.) A skeleton of 

 hollow, radial, compound, siliceous spicules, not meeting in the 

 centre ; nuclei two ; central capsule with three oscula, one of which is 

 surrounded by a mass of coloured granules (the phaeodium, from 

 which the suborder is named). Like the rest of the Phaeodaria this is 

 a deep-sea form and does not possess yellow cells. 



