64 



THE BRITISH NATURALIST. 



[March 



history and anatomy of this lowly member of the great backboned family 

 we may learn many important lessons. 



The animals collectively spoken of as Tunicates, though, on the whole, 

 of higher organization than Balanoglossus, do not possess so fish-like a 

 breathing apparatus. They breathe by a branchial sac which in propor- 

 tion to the size of the animal is usually large, and through which a 

 constant flow of water is maintained by muscular contraction. Into this 

 sac (just as is the case with the fish's mouth) the water and food is taken ; 

 the food passes through an orifice into the alimentary canal, whilst the 

 water flows through gill-slits in the sac-walls, bathing the blood vessels 

 and aerating the blood ; and ultimately is expelled from the organism 

 through the same orifice as the waste products of digestion. In the 

 lancelet, which is often spoken of as a vertebrate, the gill apparatus is 

 fairly comparable with that of Balanoglossus ; the gill-clefts are very 

 numerous, and extend backwards nearly to the middle of the body, 

 differing in this respect from all the true fishes. In the early stages of 

 development these gill-clefts open freely to the exterior, and the water 

 entering through the mouth flows out through them. In the adult form 

 two lateral folds of the skin cover the external openings of the gills, and 

 thus form pouches or sacs, which posteriorly unite and form a tube 

 which discharges the water through an orifice a little in front of the anus. 



In the true back-boned animals respiration is principally if not entirely 

 effected by the coloured corpuscles of the blood, which, mainly owing to 

 the presence of haemoglobin, possess great powers of absorbing oxygen, 

 which they take up from the surrounding medium (water or air) as they 

 pass through the delicate blood vessels of the respiratory organs (gills or 

 lungs). These coloured corpuscles are usually oval or circular discs, and 

 in the lower vertebrates they are much fewer in number, and generally 

 larger in size, than in the higher animals. The very much greater 

 number of blood discs in the higher animals, and the greater proportional 

 aerating surface offered by every one of that number, enable these 

 animals to take up a much greater quantity of oxygen, and the vital 

 processes generally are consequently much more active. It is obvious, 

 however, that the apparatus of respiration must also exert considerable 

 influence on the vitality of the organism, and that those animals which 

 derive their oxygen from the water which holds it in solution must rank 

 low or high in the scale as their gills are more or less simple. In all 

 adult fishes water is taken in at the mouth, forced over the gills, and 

 thence out again through one or more orifices. The hag, which is 

 perhaps the lowest true fish, has, externally, on either side the neck (if we 

 may use the word) seven little holes; there are corresponding holes in 

 the pharynx ; these holes are the ends of as many little pipes or tubes 

 which admit the water into slightly flattened round sacs ; these sacs are 

 lined with a delicate membrane through which the exchange of gases 



