342 FISHES CHAP. 



conditions. They appear to be more numerous in the Dipnoi 

 (e.g. Protopterus) than in any other Vertebrates, except under 

 pathological conditions. 1 



The Lymphatic System. In addition to blood-vessels, Fishes 

 possess a lymphatic system, consisting of smaller vessels, lymph- 

 capillaries or lymph-spaces, distributed in the connective tissue of 

 different parts of the body, and by their union ultimately forming 

 larger lymph-vessels or sinuses which communicate with certain of 

 the principal veins, the whole forming a series of channels for the 

 collection of the blood-plasma which has exuded from the blood- 

 capillaries for the nutrition of the tissues, and for its conveyance 

 to the general venous system. The fluid in the lymphatics, or 

 lymph, consists of dilute blood-plasma containing leucocytes but 

 devoid of red corpuscles. At the points where the larger lymph- 

 atics open into the veins, lymph-hearts may be developed. In the 

 Eel (Anguilla vulgaris) there is a lymph-heart in the tail, which 

 communicates by a valvular orifice with the smaller of the two 

 caudal veins, and by its rhythmical pulsations propels the lymph 

 into the vein. In Silurus there are two caudal lymph-hearts. 

 Apart from the lymphoid tissue, which is so abundantly present 

 in certain parts of the body, Fishes appear to be devoid of the 

 special " lymphatic glands " of the higher Vertebrates. 



The Ductless or Blood-Glands. All the important blood- 

 glands of other Vertebrates have their representatives in Fishes. 

 Nothing is certainly known of the function of these organs in 

 Fishes, but from the general structural resemblance which they 

 present to their equivalents in the higher Vertebrates, it is 

 perhaps not unreasonable to infer that they are similar in func- 

 tion. If this be so, the blood-glands of Fishes are organs for 

 leucocyte-formation and phagocytosis, involving the destruction 

 and removal of effete red blood - corpuscles ; in addition, they 

 may also be concerned with certain obscure chemical changes in 

 the composition of the blood, which have an important relation 

 to general or local nutrition. 



The Spleen. This lymphoid organ is the largest of all the 

 blood -glands, and, in the form of a compact or more or less 

 lobulated body, is present in all Fishes, and possibly in Cyclo- 

 stomes. In position the spleen is usually in close proximity 

 to the stomach, to which it is attached by an extension round it 

 1 Newton Parker, op. cit. p. 167. 



