12 



LOWER INVERTEBRATES. 



may be either spicules or a spheroidal, perforated shell. Under this division are classed 

 the very common forms Sphmrozoum, Fig. 9, and Collosphcera, the former being 

 either naked or having spicules, while the latter has a shell. 



7. Vesiculata. — To this gi-oup belong the curious jelly-like forms already men- 

 tioned, described by Huxley as Thalassicolla, Fig. 4. There is no 

 definite skeleton, but some of the species have spicules more or less 

 closely approximated. The vesiculata are distinguished by the pres- 

 ence of a nuclear vesicle, which is usually multiglobular. Formerly 

 the ThalassicoUida were classed with the CoUozoa, but the nuclear 

 vesicle is not found in the latter, and there is no external shell or 

 spicular layer in the former such as are found in Sphmrozoum and 

 CoUosphmra. 



"When we consider the wonderful symmetry, beauty, and variety 



of form revealed by a study of the hard, sUicious skeletons of the 



Polycystina, Acanthometrida, and other families of the order, we 



may well inquire how it is possible for such simple creatures to 



construct such perfect foi-ms. Mivart suggests that they may be 



produced by " a kind of organic crystallization — the expression of 



some as yet unknown law of animal organization here acting un- 



FiG. i.—Podocyrtie trammelled by adaptive modifications or by those needs which seem 



In^ed?**' ^"^^''^to be so readUy responded to by the wonderful plasticity of the 



animal world." 



Representatives of the great class Radiolaria are found in all seas, but they are by 



far the most abundant in tropical waters. The most common forms of all belong to 



the Acanthometrida and Polycystina. Their remains have formed immense beds of 



rock, mostly during the Tertiary age, but 

 they have also been found in the chalk and 

 in the Trias. They are found in the diatom- 

 aceous rock of Richmond and Petersburg, 

 Va., also in Maryland, and in Bermuda ; 

 but by far the most extensive deposits are 

 in the Nicobar Islands and the Barbadoes. 

 In the Nicobar Islands deposit they form 

 strata eleven hundred and two thousand 

 feet in thickness, in which a hundred 

 species have been identified. In the Bar- 

 badoes the rock is not quite so thick, but 

 about three hundred species have been 

 found there, most of which are Polycystine 

 forms. 



A number of minute but very beautiful 

 organisms were obtained by Mr. Murray 

 during the "Challenger" expedition by 

 using the^w-net, which undoubtedly deserve a place among the Protozoa, but they 

 have not yet been classified. They have been named Challengerida, and seem to be 

 closely related to the Radiolaria. 



iftasrozoitm ovodimaret greatly enlarged. 



tern a 



