INTRODUCTION. xxiii 



of the lower animals. The sea-anemone presents a step in advance in organs of circu- 

 lation ; here the partly digested food escapes through the open end of the stomach 

 into the perivisceral chambers formed by the numerous septa, 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 with the cilia forcing it into every 

 interstice of the body, so that the tissues are everywhere supplied with food. 



The water-vascular system of the coelenterates presents an additional step in de- 

 gree of complexity ; but it is not until we reach the echinoderms, on the one hand, 

 and such worms as the Nemertes and its allies on the other, where definite 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 eo- 

 e.vist with the water-vascular system. In the annelids, such as the Nereis, one of the 

 blood-vessels may be modified to form a pulsating tube or heart, by which the blood 

 is directly forced outward 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 molluscs have a circulatory system which presents a nearer approach to the 

 vertebrate heart and its vessels than even in the crustaceans and insects, for the ven- 

 tricle and one or two auricles, with the complicated arterial and venous system of 

 vessels of the clam, snail, and cuttle-fish, truly foreshadow the genuine heart and sys- 

 temic and pulmonary circulation of the vertebrates. The molluscs, and king-crab, and 

 the lobster, possess minute blood vessels which present some approach to the capillaries 

 of vertebrates. The circulation in certain worms, from Nemertes upward, may be said 

 to be closed, the vessels being continuous ; but they are not so in insects where true 

 veins are not to be found, the blood returning to the heart in channels or lacunre in 

 the siiaces between the muscles and viscera. 



In vertebrates the ' aortic heart ' of the lancelet or Amphioxus is simply a pulsat- 

 ing tube, and there are portions of other vessels which are pulsatile, so that there is, 

 as in some worms, a system of 'hearts.' A genuine heart, consisting of an auricle and 

 a ventricle only, first appears in the lamprey. This condition of things survives in 

 fishes, with the exception of those forms, such as the lung-fish (Dipnoans), whose 

 heart anticipates in structure that of the amphibians and reptiles, in which a second 

 auricle appears. Again, certain reptiles, such as the crocodiles, anticijjate the birds 

 and mammals in having two ventricles — i. e., a four-chambered heart. It should be 

 borne in mind that in early life the heart of all skulled vertebrates (Cr.aniota) is a 

 simple tube, and as Gegenbaur states, " as it gr.'idiuilly gets longer than the space set 

 apart for it, it is arranged in an S-shaped looj), and so takes on the form which the 

 heart has later on." Owing to this change of form it is divided into two parts, the 

 auricle and ventricle. 



A striking feature fii'St encountered in the craniate vertebrates is the presence of a 

 set of vessels conveying the nutrient fluid or chyle which filters through the walls of the 

 digestive canal to the blood-vessels ; these are, as already stated, the lymjihatics. In 

 the lancelet, as well as in the invertebr.ate animals, such vessels do not occur, but the 

 chyle oozes through the stomach-walls and directly mixes with the blood. 



Respiration. 



Always in intimate relation with the circulatory system are the means of respira- 

 tion. The process may be carried on all over the body in the simple animals, such 



