BRIEF REVIEW OF THE SYSTEM OF LIFE. 



437 



having siliceous shells are others. A few species are represented in Figs. 487-493. 

 Another group is that of Desmicls, which consist of one or a few greenish cells, and secrete 

 little or no silica. They are related to the common Conferva (frog-spittle) of fresh-water 

 pools. Other calcareous kinds take delicate branching forms, as the Corallines ; or more 

 stony forms, like those of Corals, but destitute of surface cells, as the Nullipores ; or 

 sponge-like or concretion-like forms, as the cal- 

 careous Algse of the Yellowstone Park. Some 

 related to the last-mentioned occurring in warm 

 waters secrete silica. There are also the minute 

 Coccoliths over the ocean's bottom in deep or 

 shallow waters ; they are so named from the 

 Greek for seed and stone. 



488-493. 



487. 



.% L 



Figs. 487-493, Diatoms highly magnified; 4S7, A group of fossil Diatoms; 488, Pinnularia peregiina, 

 Richmond, Va. ; 489, Pleurosigma angulatum, id. ; 490, Actinoptychus senarius, id.; 491, Melosira sul- 

 cata, id. ; a, transverse section of the same; 492, Grammatophora marina, from the salt water at Stoning- 

 ton, Conn.; 493, Bacillaria paradoxa. West Point. 



The common leathery seaweeds of the seacoast, or the Fucus division, include the 

 Sargassum of the Atlantic, the air-cavities in which enable it to float. 



CEPHALTZATION, A GENERAL PRINCIPLE BEARING ON SYSTEM 

 AND GRADE IN THE ANIMAL KINGDOM. 



Since an animal has, typically, an anterior nervous mass or ganglion determining the 

 position of the head, and antero-posterior conditions in growth, a greater or less subordi- 

 nation to the head in the arrangement of its organs should naturally be looked for. Degree 

 of structural subordination to the head and of concentration headward in body-structure, 

 is referred to under the term Cephalization. 



The principle is illustrated in the class of Crustaceans, with special clearness and 

 large distinctive characters, on account of the fewness of the species and their size. 



Some preliminary explanations are here first given respecting Worms, and then the 

 facts from the class of Crustaceans. 



1. Worms. — An example of a low decephalized condition among Articulates exists 

 in the Tape-worm, Taenia solium, one of the lowest of the so-called Worms. It grows and 

 elongates by the multiplication of segments (by budding), until their number is sometimes 

 several hundred, the new segments forming successively just behind the head. The head 

 has its very small nervous ganglion, from wliich slender nerves pass backward ; so that in 

 growth and nerves it is an individual. But it has no mouth, and the body no stomach or 

 intestine. Instead of this, each segment is so far complete in its individuality that it takes 

 its independent nutriment, and has its own reproductive system ; and if separated from 

 the rest of the series, it has all that is required for propagating the species by ova. Here 



