BRIEF REVIEW OF THE SYSTEM OF LIFE. 



433 



Globigeiinte, with Diatoms, from 

 a deposit off Alligator Reef 

 (x 15). A. Agassiz. 



Fig. 461 is a 1-celled species ; the others are compound, and contain a number of 

 "exceedingly minute cells. A few are comparatively large species, and have the shape of 

 a disk or coin, as Fig. 473, a Nummulite, natural size ; the figure shows the interior cells 

 of one half : these cells form a coil about the center. Orbitoides is the name of another 

 genus of coin-like species. Fig. 474 a is a species of Ficsic- 

 iina, a kind nearly as large as a grain of wheat, related to 475. 



the Nummulites ; 474 6 is a transverse view of the same. 

 This is one of the ancient forms of Rhizopods, occurring in 

 the rocks of the Coal formation. Rhizopods of the genus 

 Globigerina and other forms have been already mentioned 

 (page 144) as the chief constituents of the calcareous ooze 

 or mud making much of the sea-bottom. Fig. 475 repre- 

 sents an aggi'egation of Globigerinae with Diatoms, found 

 at a depth of 880 feet, off Alligator Reef, near the south 

 coast of Florida, as figured by A. Agassiz. 



Each Rhizopod cell is occupied by a separate animal or 

 •zooid, though organically connected with the others of the 

 same group or shell. The animal is of the simplest kind, 

 having no mouth or stomach, and no members except slen- 

 der processes of its own substance, which it extrudes through 

 pores in the shell, if it have any. The name Bhizopods comes from the Greek for root- 

 like feet, — in allusion to the root-like processes they throw out. Some of the species not 

 secreting shells (as in the genus Amoeba) have been seen to extemporize a mouth and 

 stomach. -When a particle of food touches the surface, the part begins to be depressed, 

 and finally the sides of the depression close over the particle, and thus mouth and stom- 

 ach are made when needed ; after digestion is complete, the refuse portion is allowed 

 to escape. 



The shells of some Rhizopods do not con- 

 •sist of distinct cells : the aggregate living mass 

 secretes carbonate of lime, without retaining 

 the distinction of the zooids. This is the case, 

 as Carpenter has observed, in the Nummulite- 

 like genus Orbitolites. Some species make 

 large coral-like masses instead of small 

 shells. 



2. Radiolarians (Polycystines). — Se- 

 crete siliceous shells which are symmetrically 

 radiate or circular. Three species, from the 

 Barbados, are represented in Figs. 476 to 



478. Fig. 476, Lychnocanium lucerna Ehr. ; Fig. 477, Eucyrtidium Mongolfieri Ehr, 

 Fig. 478, Halicalyptra fimbriata Ehr., the first two magnified 

 100 diameters, the last about 75. From these deeply concave 

 forms, there are gradations in one direction to disks with con- 

 cave centers, and to flat disks, both with plain and pointed 

 borders, and in the other direction to elongate, conical, and 

 spindle-shaped forms. Others have the shape of a flattened 

 •cross; another is an open diamond, with narrow diagonals and 

 periphery. The disks have a concentric, and not a spiral, 

 .structure, and thus are unlike those of Nummulites. 



The annexed figure represents a minute spherical species 

 of Radiolarian — a jelly-like globule bristled with spicules — 



which sometimes beclouds the water in the Pacific and East Indian seas (Sphcerozoum 

 ■orientale D.). 



DANA'S MANUAL — 28 



476. 



477. 



478. 



476, Lychnocanium lucerna (x 100) ; 477, Eucyr- 

 tidium Mongolfieri (x 100) ; 478, Halicalyptra 

 fimbriata (x 75). 



479. 



