is 



ILLUSTRATED NATURAL HISTORY. 





P <21 



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STARFISH : DIVISION ItADIATA. 



or of aggregations of similar independent cells ; but in many instances their bodies and organs arc 

 constructed entirely of a gelatinous cellular matter very like that of which the Protozoa are com 

 posed, and which appears to possess almost an equal power of retaining vitality in its smallest 

 particles. As wc advance in the group, however, we find the organization of its constituent ani- 

 mals growing more and more complicated, from the vital functions becoming more and more 

 differentiated — that is to say, performed by organs specially devoted to each ; until, from creatures 

 roughly shaped out of a homogeneous semi-gelatinous mass, we gradually arrive at animals fur- 

 nished with distinct nervous and vascular systems, with organs of motion and reproduction. 



The most striking character of the animals included in this group consists in the radiate arrange- 

 ment of their organs, as in the star-fish, round a central axis, which generally passes through tic 

 mouth. From this peculiarity they have been denominated by zoologists radiated animals, and 

 constitute the division Radiata. This group includes those animals which were formerly supposed 

 to approach very closely to plants, or indeed rather to partake of a sort of mixed nature intense 

 diate between animals and vegetables, hence called zoophites, or animal-plants ; and some author 

 still make use of this name in preference to that of Radiata, to indicate the present group. 



The nervous system can only be recognized distinctly in the most highly organized of thes< 

 animals. In these it partakes of the radiate arrangement of the body, the nerve distributed t< 

 each division of tin; body corresponding exactly with those of its neighbor, and arising from ; 

 separate center. These centers are all placed in a circle round the mouth, and united by a con 

 which forms a complete ring. 



The sense of touch appears to be the only one which can with certainty be ascribed to the?' 

 animals; this resides in the general covering or integument, and is also frequently exercised b; 

 special organs. 



All tin- Radiata po — 9 a month and intestinal cavity; but very few of them have a secon< 

 opening for the discharge of fecal matters. They generally possess a more or less distinct vascula 

 system : in some of the higher forms a sac-like heart occurs. 



Sexual reproduction occurs in all the Radiata, and the sexes are generally on separate indivkl 

 mils. Propagation is also very commonly effected in this division by the formation of buds o 

 gemmules; and these either remain attached to the parent stock, which thus "goes on increasing 

 continually in size, or become free, and lead an independent existence. 



In the two preceding divisions of the animal kingdom wc find the body formed upon two ver 



