CIRCULATION. TEETH. 



plagiostomes and a continuation of the ventricle having striated muscular 

 fibres, internally it possesses no valves at its base, but is lined by several 

 rows, while it regularly contracts, alternating with the contractions of the 

 ventricle. Internally it is provided with transverse rows of longitudinally 

 shaped valves, varying in number with the forms to which they pertain : 

 among the Chondropterygii it has been termed conus arteriosus to dis- 

 tinguish it from what is seen in teleosteans and cyclostomes. From the 

 arterial bulb, or rather a continuation of it, is the branchial artery, which 

 sends off branches from either side to the gills, where the blood having 

 been purified, is returned to the dorsal vessel or aorta which distributes it 

 throughout the body. It will be unnecessary in this place to remark any 

 further on the arterial, venous, or lymphatic systems in fishes. 



If we except the aberrant form of Amphioxus, all fishes possess blood 

 corpuscles, varying considerably in size, being largest in the lepidosiren, of 

 rather less size in the plagiostomes, while among the bony fishes those of 

 the Salmonidas appear to be of the greatest diameter, but are inferior 

 in size to those of the Chondropterygians. It contains red and colourless 

 globules, the former being as a rule elliptical, but are round in lampreys ; 

 circular globules are not unfrequently perceptible in the blood of fishes. 



TEETH. 



The teeth of fishes are more generally concerned in capturing than in 

 masticating their prey, and are vained in their form, sometimes differing with 

 the age or sex of the individual, and found in various situations inside the 

 mouth and contiguous parts, thereby furnishing an important aid in 

 classification, assisted likewise by their external characters. Some forms 

 are destitute of teeth, or possess them solely in the pharyngeal bones, but 

 from the lips to all the bones entering more or less into the composition of 

 the buccal cavity, there is a tendency of the mucous membrane in almost 

 any of these parts to develop teeth, especially among teleosteans. This 

 is also seen iu some plagiostomes. Among osseous fishes teeth are 

 most commonly observed springing from or more or less attached to 

 bone. Teeth as a rule are simple, and may be isolated one from 

 another, as in the porbeagle shark (plate clvi), or they may be com- 

 pound and form a large plate with pavement-like subdivisions, as seen in 

 Myliobatis (plate clxxvi) . Among teleosteans they may appear as a bony 

 continuation of the jaws, as in Tetrodons (vol. ii, plate cxlvii). In some, as 

 carps, teeth are absent from the jaws, or they may be isolated, as in the 

 anterior portion of the jaws in the wolf- fish (plate lviii), or in a single row, 

 as in the blennies, a double row, as in the holibut (plate xciv), a treble row, 



