24 COMPARATIVE ANATOMY. 



ticulated segments, is placed externally to the soft parts, as in 

 Articulata, or arranged to them as a central longitudinal axis, 

 as in Vertebrata. The compact variety generally conforms 

 to the type to which the animal belongs. Thus, in Radiata 

 it is rayed, in Mollusca the mantle is calcified in one or more 

 plates, and in the Articulata and Vertebrata the parts and 

 accessories of the ring and vertebra are respectively main- 

 tained. 



The skeleton may arise from animals whose tissues are 

 not otherwise specialized, while in those highly organized it 

 is often absent. Thus, Nanomia, a foraminiferous rhizopod, 

 possesses an elaborate skeleton, while with Doris, a gas- 

 teropod mollusc, a few calcareous spicules are alone present. 

 Within a natural group it may be absent (Amceba), diffuse 

 (Thallasicolla, Sphasrozoum), or compact (Foraminifera). 



The skeletons of composite animals, as Polypi and Polyzoa, 

 resemble one another more or less intimately. 



The presence of the skeleton is determined by habit. 

 Lowly organized animals, if floating (Aurelia, Physalia), often 

 have no skeletons; but if fixed (coralline polyps) or moving 

 upon a ground surface (Echinodermata), they are generally 

 present. 



Skeletons are of various composition, viz. : calcareous, cor- 

 neous, chitinous, siliceous, cartilaginous, or osseous.* 



PROTOZOA. In Foraminifera the sarcodous mass is con- 

 tained within a chambered shell, having perforate walls and 

 partitions, successive cells being added by a process of gem- 

 mation. The form of the shell is subject to great variety, 

 the prevalent shapes being modifications of the disk or spiral.f 

 In Eozoon, an extinct genus of Protozoa, allied to the Fora- 

 minifera, the skeleton is deposited between layers of sarcode 



* A ' false skeleton ' is an agglutination of extraneous matter fixed to 

 the animal by cutaneous secretion, as in Terebella. 



t So minute are these shells that it is estimated that 6000 can be counted 

 in a grain, and 3,800.000 in an ounce of forarainiferous sand. The pyramids 

 of Egypt are built of what is known as nummulitic limestone, so called from 

 the bulk of the etone being chiefly due to the presence of enormous num- 

 bers of a species of Foraminifera (Xummulus). 



