10 



LOWER INVERTEBRATES. 



losphcfra, are oval, about .008 mm. in lengtli, and liave at least one eilium. The sub- 

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

 like Cvllosphmra 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 simjjle 

 ])lan 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 

 structure will 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- 



Fia. 6. — Haliomma polijacanthum^ 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. — Diseoidal forms with skeletons partly intra-capsular, generally foi-m- 

 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 parts is quite strik- 

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

 species. 



