576 COMPARATIVE ANATOMY. 



system. In any case the whole apparatus agrees only generally 

 with the vascular system of the Craniota, and it may be regarded 

 as an expression of the great difference which there is between 

 these two divisions, as shown by many other characters. 



The blood-fluid of Amphioxus is colourless, and its form-elements 

 are very small, indifferent, cells. 



§ 430. 



The Craniota possess a single organ, the heart, which effects the 

 circulation of the nutrient fluid ; they are also distinguished by a 

 differentiation of the circulatory vessels, which is of such a kind that 

 the nutrient fluid itself may be divided into two categories. Part 

 of the blood which passes through the body into the tissues, and 

 which, during the circulation, filters through them, is collected into 

 special tracts, and is again restored to the principal stream. This 

 fluid is the lymph, and its vessels form the lymphatic system. 

 The vessels which pass directly from the heart, and which lead back 

 again to it, form the blood-vascular system. The lymph-vessels 

 which are distributed in the wall of the enteron take up the 

 formative material, or chyle, which has been formed by the process 

 of digestion, and carry it into the blood-stream. They make up, 

 therefore, for the loss which the blood has suffered in conse- 

 quence of the metastasis which is continually taking place in its 

 circulation through the body. The lymphatic system is an im- 

 portant part of the whole vascular system. That system gains an 

 important step by this elaboration, and, owing to its division into these 

 two parts, is seen to be more highly developed than are the arrange- 

 ments which obtain in the Acrania, and in all the Invertebrata. 



The separation of the nutrient fluid into blood and lymph is 

 accompanied by a differentiation of the form-elements of these 

 fluids. Those of the lymph are indifferent cells, which are similar 

 to the blood-cells of most Invertebrata. The form-elements of the 

 blood are, in their earliest stages of development, similar in cha- 

 racter, but are afterwards converted into coloured corpuscles of a 

 definite form, although this form is different in different divisions. 

 Owing to their large quantity they give the blood the appearance of 

 being coloured, as compared with the colourless lymph. 



In all points but their size the lymph-cells of the Vertebrata 

 are similar to one another. The blood-cells, however, which are 

 much more differentiated, differ a good deal. They are all cellular 

 in character, so far as this depends on the nucleus, although, 

 indeed, it is only present in Mammals during- their foetal stages. 

 As a rule, the blood-cells are flattened and discoid. In Fishes, 

 Amphibia, Reptiles, and Birds, they are oval and biconvex, 

 for the centre of each surface protrudes slightly. They form 

 biconcave rounded discs in the Mammalia, but in some they are 

 oval (e.g. Tylopoda). In size, those of the Dipnoi and Amphibia 

 (especially Proteus, Siren, etc.) arc the largest. Owing to the 



