COMPARATIVE ANATOMY OF ORGANS. 635 



Organs of Circulation. Intimately associated with the 

 digestive canal are the vessels in which the products of di- 

 gestion mix with the blood and supply nourishment for the 

 tissues, or. in other words, for the growth of the body. In 

 the Infusoria the evident use of the contractile vesicles is to 

 aid in the diffusion of the partly digested food of these mi- 

 croscopic forms. In the Hydra the food-stuff is directly 

 taken up by the cells lining the coelum, while the imper- 

 fectly formed blood also finds access to the hollows of the 

 tentacles. The mode in which the cells lining the canals 

 in the sponge take up, by means of the large cilia, micro- 

 scopic particles of food, directly absorbing them in their 

 substance, is an interesting example of the mode of nourish- 

 ment of cellular tissues of the lower animals. 



The sea-anemone presents a step in advance in organs of 

 circulation ; here the partly digested food escapes through 

 the open end of the stomach into the perivisceral chambers, 

 the action of the cilia, with the contractions of the body, 

 churning the blood, consisting of sea- water and the particles 

 of digested food, and a few blood-corpuscles, hither and 

 thither, and forcing it into every interstice of the body, 

 even into the tentacles, so that the tissues are everywhere 

 supplied with food. 



The water-vascular system of the Ccelenterates presents an 

 additional step in degree of complexity ; but it is not until 

 we reach the Echinoderms on the one hand, and such 

 worms as the Nemertes and allies on the other, where defi- 

 nite tubes or canals, the larger ones contractile, and in the 

 latter type at least formed from the mesoderm, serve to 

 convey a true blood to the various parts of the body, that 

 we have a definite blood system. In the Echinoderms a 

 true haemal or vascular system may co-exist with the water- 

 vascular system. In the annelids, such as the Nereis, one 

 of the blood-vessels may be modified to form ix pulsating 

 tube or " heart," by which the blood is directly forced out- 

 ward to the periphery of the body through vessels which may, 

 by courtesy, be called arteries, while the blood returns to 

 the " heart " by so-called veins. 



The mollusks have a circulatory system which presents a 



