62 



ANIMAL STUDIES 



tion of food, and extending deep into the tissues of the 

 body, carries nutriment to otherwise isolated regions. In 

 the fresh-water forms and their allies there are three main 

 branches of the intestine (Fig. 33), while in many of those 

 from the sea there are several, and their arrangement 

 affords a basis for their general classification. 



55. Excretory system. In the sponges and ccelenterates 

 the wastes are cast out by the various cells into the gastric 

 cavity or at once to the exterior with- 

 out the aid of any pronounced system 



of vessels ; but in the flatworms sev- 

 eral of the organs are deeply buried 

 within the tissues of the body and a 

 drainage system becomes a necessity. 

 This consists of a paired system of ves- 

 sels extending the length of the ani- 

 mal (Fig. 33) and provided with numer- 

 ous branches, some of which open at 

 various points on the surface of the 

 body, while the others terminate in 

 spaces (Fig. 34, s) among the organs in 

 what are known as flame-cells. The 

 substances which accumulate in these 

 spaces are gathered up by the flame- 

 cell, poured into the space it contains, and by means of the 

 vibratory motion of its flagellum (/), a movement bearing 

 a fancied resemblance to the flickering of a flame in the 

 wind, are borne through the tubes to the exterior. 



56. Nervous system and sense-organs. In the sponges no 

 definite nervous system is known to exist, the slight move- 

 ments which the cells are able to undergo being regulated 

 somewhat as they are in the Protozoa. Among the ccelen- 

 terates certain of the cells scattered over the surface of the 

 body are set aside as nerve-cells, and, more or less united by 

 means of fibers extending from them, convey impulses over 

 the body. In the flatworms the larger number of nerve-cells 



FIG. 34. Flame-cell of flat- 

 worm (after LANG). /, 

 flagellum ; n, nucleus; 

 s, spaces among the or- 

 gans of the body ; v, 

 waste materials. 



