44 HIGH SCHOOL ZOOLOGY. 



miis, Iti some fishes by the itnion of the gill-cover to the skin 

 over the pectoral arch, this aperture may be much reduced in 

 size. 



58. In the roof of the branchial cavity in front of the pectoral girdle, 

 an organ, the tliymus, is present in the fishes which attains a considerable 

 size in some forms, and is difficult to make out in others. It is seen in the 

 young catfish, but not in the adult. Again, below the floor of the mouth- 

 cavity, round the origin of the vessels which ascend the gill-arches, is 

 another organ, the thyroid, which is of some size in the adult. The 

 functions of both of these structures are obscure, but the organs are 

 found in similar positions in all Vertebrates. 



59. Vascular System. — In all Vertebrates two subdivi- 

 sions are present, the blood- and the lymph-vascular system. 

 The former embraces the heart and blood-vessels and their 

 contents, the latter the lymph- vessels and spaces and the 

 lymph-glands in which cei'tain elements of the lymph are formed. 



60. Both the circulating fluids contain corpuscles, which in 

 the blood are of two kinds, coloured and colourless, while in the 

 lymph only the colourless corpuscles are present (Fig. 3, a). The 

 coloured corpuscles while passing through the fine vessels of the 

 gills, have a remarkable power of combiniug with the oxygen 

 contained in the water which bathes the gills, V)ut they just as 

 easily gi^-e up this oxygen to the other tissues which require it. 

 This power they owe to the haemoglobin, which they con- 

 tain, and which also is the cause of their colou)-. The amcB- 

 boid colourless corpuscles have, on the other hand, a faculty 

 which the coloured ones do not possess to any extent, that of 

 changing their shape and incorporating foreign particles. 



61. The centre of the blood- vascular system is the heart; 

 from it in front are given off the arteries, thi-ough which the 

 blood is distributed by way of the gills to the body generally, 

 and towards it behind, the veins converge, through which the 

 blood is returned to the heart to be asrain sent on its course. 

 Between the arteries and the veins are the finer capillary 



