THE CIRCULATION IN NON-VERTEBRATA. 695 



ber and wide distribution of rhythmically contractile cavities in the circulating- 

 system of the Amphioxus, suggest a comparison with the multiple hearts, or 

 pulsating chambers, seen in certain Molluscous and Annulose animals. Its 

 blood also contains only colorless corpuscles. 



In the Non-vertebrate animals, the blood, and the organs of its circulation, 

 where they exist, are peculiar, and distinguished from the same parts in the 

 Yertebrata. The blood, in the higher forms of these animals, is corpuscu- 

 lated, but its corpuscles are not smooth and colored with cruorin, like the red 

 corpuscles of the Vertebrata, but, even when the blood itself is tinged, they 

 are usually colorless and granular, like the white blood-corpuscles and lymph 

 corpuscles of the Vertebrata. Sometimes, however, they are flattened, dis- 

 coid, or even angular ; and occasionally they have a bluish or greenish tint. 

 In the simplest of these animals, the blood contains only granules. 



This common nutritive fluid, or blood, has, indeed, been regarded by some 

 as corresponding rather with the lymph, and the vessels in which it circu- 

 lates, as homologous rather with the lymphatic system of the Vertebrata. It 

 is certain that no other vessels corresponding with lymphatics, exist in the 

 Non- vertebrate animals ; and no lymphatics have been detected in the Am- 

 phioxus, which has only colorless corpuscles. The vessels in which it circu- 

 lates, present this great peculiarity as contrasted with the circulating appara- 

 tus of the Vertebrata, that they no longer form a continuous series of well- 

 defined tubes, or true bloodvessels ; but, even in the highest forms, and, to a 

 still greater degree, in the lowest forms, the blood passes from such definite 

 vessels into spaces lying, as it were, between the viscera and tissues of the 

 body, furnished doubtless with a lining membrane, but not having otherwise 

 distinct walls or coats. In the higher of these animals, these spaces form 

 spongy recesses, sinuses, or lacunas, and in. the lower forms, merely perivisceral 

 or intervisceral spaces. 



Connected with the true vessels, in many of the higher forms, there are 

 found one or more rhythmically pulsating contractile cavities, which are there- 

 fore called hearts ; sometimes these are unilocular or ventricular, sometimes 

 bilocular, or auricular and ventricular ; sometimes they are multilocular. One 

 of these hearts only can be regarded as homologous with the heart of the Ver- 

 tebrata, in some of which animals, as in the bat, the eel, torpedo, myxine, 

 and amphioxus, for example, portions of the venous system are also pulsatile. 

 Pulsating cavities also exist, as we have seen, in certain Reptiles, Amphibia, 

 and Fishes, in connection with the lymphatic system, viz., the lymphatic 

 hearts. 



When the heart in a Non-vertebrate animal is single, it is, as a rule, placed 

 on the dorsal aspect of the body, I. e., on the surface opposite to the one upon 

 which the nervous axis is found ; it is, moreover, systemic, or sends the blood 

 to the various parts of the body, including the liver, and not to the respiratory 

 organs, which only receive the blood as it is returning from the body to the 

 heart. This is the reverse of the arrangement met with in Fishes, in which 

 the heart is respiratory. In the Fish, the heart receives and transmits deoxy- 

 genated blood, whilst in the Non-vertebrate animal it receives and transmits 

 an oxygenated fluid. When the heart is bilocular, it is constricted externally, 

 and presents a delicate projecting border or valve between the auricle and 

 ventricle. When the heart is multiple, the supernumerary ones are always 

 simple or respiratory, and are placed upon the veins near the respiratory or- 

 gans, any modifications of which, as in the Vertebrata, necessitate changes 

 in the arrangements of the vascular system. A portal venous system never 

 exists, the liver being always supplied by the systemic arteries. The vessels 

 are named arteries and veins, according to the direction of the current within 

 them, and not from their structure ; for distinct coats, like those of the arte- 

 ries in the Vertebrata, are not recognizable. Though extremely delicate valves 

 are present in the heart, they are not found so regularly disposed in the veins, 

 as in the Vertebrata. Sometimes a pericardium exists. True capillaries, 

 provided with distinct walls, and intermediate between the finest arteries and 

 the finest veins, have not been demonstrated, and in most instances are cer- 

 tainly absent ; but, the arteries pour out the circulating corpusculated fluid, 



