38 INVERTEBRATE ANIMALS. 



a distinct formation. It attains in places a thickness of several thousand 

 feet, and is especially largely developed in the Alps. It has an historic in- 

 terest from the fact that the Pyramids are built of it, and that the Nummu- 

 u'tes in it were noticed by Herodotus, the " father of history." 



FIG. 5. Nummulites Ic&vigatug. 



ORDER IV. RADIOLARIA (Lat. radius, a ray). The third 

 order of the Ehizopoda is that of the Radiolaria, essentially 

 distinguished by the fact that the sarcode-body has the power 

 of secreting a " siliceous " or flinty skeleton, either in the form 

 of a shell, or of detached spicules or needles ; while the pseu- 

 dopodia are long and thread-like, and stand out from the body 

 like rays. In this last character the Radiolaria approach 

 very closely to the Foraminifera ; and the resemblance be- 

 tween the two groups is still further increased by the fact 

 that the pseudopodia often run into one another so as to form 

 a net-work, and sometimes show a circulation of granules 

 along their edges. Three groups of organisms have been 

 described as belonging to the Radiolaria, and we may briefly 

 notice an example of each of these. 



In the first family we have organisms like Acanthometra 

 (Fig. 6, ), in which the body is composed of sarcode, sup- 

 ported by a framework of siliceous or flinty rods, which all 

 meet in a common centre. The spines or rods are all perfo- 

 rated by canals, and each conveys a pseudopodium, which is 

 protruded from an aperture at its apex. Many pseudopodia, 

 however, are given off from the surface of the body directly, 

 and are not enclosed in the spines. The Acanthometrce are 

 all minute, and are found floating near the surface in the open 

 ocean, sometimes in great numbers. 



In the second family (Polycystina, Fig. 6, b) we have a 

 number of beautiful little organisms closely allied to the Fo- 

 raminifera, but differing in the fact that the body is enclosed 

 in a glassy shell composed of flint. The shell is perforated 

 by numerous holes through which the pseudopodia are 

 emitted, and it is usually of extreme beauty, being sculptured 



