10 



LOWER INVERTEBRATES. 



losphcera, are oval, about .008 mm. in length, and have at least one cilium. The sub- 

 sequent history of the zoospores has not been made out. It is probable that colonies 

 like CoUosphcera are formed by division of this kind. 



In this large and exceedingly interesting order of Rhizopods, there are nearly a 

 thousand species, about one-half of them being fossil forms. This shows the wonderful 

 variety of form which such organisms may present without departing from the simple 

 plan or organization which characterizes them. They may be classified according to 

 the forms of their skeleton, into families and sub-families, in which one general plan of 

 structui-e wUl be characteristic of each division, as Dr. Wallich has shown. Although 

 such a classification may be convenient, it throws but little light upon the physiological 

 or morphological relations of the different forms. In the present state of our knowl- 



FiG. 6. — Haliomma polyacanthum, magnified 200 times. 



edge of the Radiolaria no fully satisfactory classification is possible. Perhaps the best 

 yet proposed is that of Prof. Mivart, which is a greatly modified form of Haeckel's 

 comprehensive but confusing plan. Prof. Mivart arranges all the Radiolaria under 

 seven divisions, which may be briefly characterized as follows : — 



1. DisciDA. — Discoidal forms with skeletons partly intra-capsular, generally form- 

 ing an external perforated shell with an internal partition, making a series of connect- 

 ing, concentric, or spirally arranged chambers ; no nuclear vesicle. 



In this group there are five sub-divisions, but the most common form of all is the 

 Astromma, in which the combination of radial and circumferential pai-ts is quite strik- 

 ing, both for beauty and for the great variations in form manifested by the different 

 species. 



