EECKXT RESEARCHES THE RADIOLARIA. 



147 



sheaths of sarcode which in life envelop the spines to their 

 apices. At the summit of each such blunt prominence there is 

 a circle of small papillae, which consist of the remains of the re- 

 tracted pseudopodia. 



As has been said, a system of internal hard parts is more or 

 less developed in almost all Eadiolarians. The few yet known 

 utterly devoid of a skeleton are the simple forms ThalassicoUa, 

 ThalassoIamj)e, and Myxohrachia*, and the compound form Collo- 

 zoum. The skeleton consists generally of silex only, and is never 

 calcareous t. In some forms, however, it consists only of a 

 peculiar cartilaginous animal substance " «^?awM^?2 " J. In some 

 forms this acanthin becomes, with age, more or less replaced by 

 silica. 



The form of the skeleton varies greatly, from extreme simplicity 

 to extreme complexity. It may be described as consisting of two 

 systems of parts : — 



A. A system of circumferential (tangential) parts ; 



B. A system of radiating parts. 



Either of these may exist (alone or with the other) in different 

 degrees of development, from the most rudimentary condition up 

 to an extreme degree of complexity. 



These parts may also both exist in so fragmentary a state and 

 in such a complex entanglement, as to form a spongy skeletal net- 

 work which may coexist with simpler parts of either of the two 

 skeletal systems or by itself alone. Thus the spongy network, if 

 it were considered a third kind of skeleton, might be said to pass 

 gradually either into the circumferential or into the radial system 

 of parts. 



Both systems of parts may exist, in different groups of Eadio- 

 larians, either externally to or more or less within the capsule, or 

 both within and without it simultaneously. 



Both the circumferential and the radial parts may be either 



* I do not regard the calcareous formation found in this genus as 

 skeletal. 



t Sir C. Wyville Thomson has (as before noted) described shortly and jSgured 

 a Radiolarian, Calcaromma calcarea, in which calcareous spheres like " the 

 spicules of a Holothurian " were found. Until we have more detailed infor- 

 mation, I hesiUite as to the truly skeletal nature of these calcareous particles. 

 See 'Voyage of the Challenger,' toI. i. p. 233, fig. 51. 



X A substance much like keratin. It is eaten into and destroyed by sulphuric 

 acid. 



