Glass 2. Pisces. 377 



ill the Teleostei, is much swollen, and provided with thick, 

 walls just at the point of origin. This enlargement, the bulb us 

 arteriosus,* is whitish like the other arteries, and contains 

 simply smooth muscle-cells, whilst the conus, with which it was 

 until recently included, is red, and possesses striated muscle- 

 cells. The cardiac aorta sends a branch to each gill-bearing 

 arch; if the opercular gill is well developed, a branch also goes 

 to this, but not if it is rudimentary. These branches, the 

 afferent branchial arteries, run from below upwards, along 

 the hinder edge of the gill-bar, and give to each gill lamella a twig,, 

 which breaks into capillaries. From each lamella, there arises, again, 

 a small vessel, which, with those like it from the same visceral 

 arch, forms an efferent branchial arter y.t This runs near 

 to the afferent artery, and unites dorsally with the corresponding 

 vessels from other visceral arches, to form the aorta, which runs 

 backwards, just beneath the vertebral column, and gives off branches 

 to various parts of the body. All the veins flow into the sinus 

 venosus, which opens into the auricle. The blood entering the 

 heart is thus venous, reaches the gills in this condition, becomes 

 arterialised there, and thence flows into the arteries. 



Thei-e is, tlierefoi-e, a complete separation of arterial, from venous, blood in 

 Pisces, and the condition of tlie vascular system accords witli the general plan, 

 given on pp. 28, 29. Some Fish, however, which possess other respiratory organs- 

 as well as gUls, form an exception, for in them the arterial and venous blood is. 

 more or less mixed. In Lepidosteus, for example, the lung receives from the 

 aorta, arterial blood, to be further oxydised, whilst the pulmonary veins, which 

 thus carry blood very rich in oxygen, unite with the large veins, bringing venous 

 ■ blood from the rest of the body. The heart and giUs thus receive mixed blood. 

 In the Dipnoi, where the lung, as in higher Vertebrata, receives blood from 

 the last arterial arch (the last efferent branchial vessel), there ai-e special 

 contrivances to partially remedy the defect, but these are too complicated to be 

 gone into here. 



The piscine kidneys are usually elongate organs, and in many 

 Teleostei, in which they lie above the swim-bladder close against 

 the vertebral column, extend]: the whole length of the body from 

 head to tail, and are often united§ behind. In the Selachians and 



* A similar swelling is present in Amia, but in no other Pisces excepting the 

 Teleostei ; whilst in this genus the wall is little thickened. 



t These efferent branchial vessels are frequently, but incorrectly termed, 

 branchial v-eins, and the afferent vessels simply distinguished as "branchial 

 arteries." In some Fish, instead of one, two efferent vessels may arise from each gill- 

 bar. 



J The most anterior portion of this long kidney is the persistent pronephros, which 

 is frequently very large in the adult, but has usually not an excretory function. 



§ A very interesting modification of the kidney occurs in the male Sea Stickleback 

 (Spinachia vulgaris), which binds various foreign bodies together by fine mucous, 

 threads, and thus forms a nest for the eggs. The mucus of which these threads, 

 consist is maniif aotured in the kidneys; some of the gland cells of the 

 urinary tubules are modiiied to secrete mucus, and are of different appearance from the 

 rest of the cells. 



