390 THE DOGFISH less. 



With regard to the general morphology of the blood-system, 

 the dorsal aorta with the caudal artery may be considered as 

 a dorsal vessel (compare Polygordius, p. 279, and Crayfish, 

 p. 340), the caudal vein, hepatic portal vein, heart, and ven- 

 tral aorta as together representing a ventral vessel, the affer- 

 ent and efferent branchial arteries as commissural vessels, 

 and the lateral veins as lateral vessels. It will be seen that 

 the heart of Vertebrates is a muscular dilatation of the 

 ventral vessel. 



The blood is red, the colour being due, as in some species 

 of Polygordius (p. 280), to haemoglobin. The pigment is not, 

 however, diffused in the plasma of the blood, but is confined 

 to the red corpuscles^ flattened oval cells with large nuclei, 

 like those of the frog referred to in an early Lesson (p. 56, 

 Fig. 8). Among the red corpuscles, but in much smaller 

 numbers, are leucocytes. When the blood is fully oxy- 

 genated it takes on a bright scarlet colour, and is usually 

 called arterial blood ; when the oxygen has been given up to 

 the tissues the colour becomes dull purple, and the blood is 

 called venous. But the student must avoid the common 

 error that arterial blood is necessarily confined to arteries 

 and venous to veins ; in the dogfish, for instance, the ventral 

 aorta and the afferent branchial arteries contain venous 

 blood. 



In addition to the blood-vessels the dogfish possesses a 

 set of channels called lymphatics (Fig. 104A, ly\ consisting of 

 colourless thin -walled vessels, mostly running alongside the 

 arteries and vems. Traced in one direction they ramify ex- 

 tensively, and end in a set of lymph-capillaries interwoven 

 with, but distinct from, the blood- capillaries ; traced in the 

 other direction they join into larger and larger trunks, pro- 

 vided at intervals with valves, and finally open into the 

 veins. The lymph- capillaries take up the drainage from the 



