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.’ 
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 Stlurus 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 fune- 
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. 
