CLASSIFICATION OF FISHES. 27 



(as in Osseous Fishes) ; and those having them fixed. The 

 former group constitutes but a single order Sturion*s but the 

 latter is again subdivided into those which have the jaws move- 

 able and adapted for mastication, Plagiostomi; and those which 

 have them soldered into a ring for suction. The latter are termed 

 Cyclostomi, or circular-monthed Fishes. 



573. In the system of Miiller above referred to, the primary 

 division of the class of Fishes in accordance with the nature of 

 the skeleton is done away with, as it is found that, by adopting this 

 as the first point in classification, nearly allied Fishes are often 

 widely separated ; and this is especially the case when the Fossil 

 Fishes are taken into the account. The classification of Pro- 

 fessor Miiller admits five primary groups of Fishes,* which he 

 calls sub-classes, but which the majority of subsequent writers 

 have agreed to regard as orders. Of these the first four unmis- 

 takeably exhibit all the characters of vertebrated animals, pos- 

 sessing a distinct vertebral column either of a bony or cartila- 

 ginous texture ; a spinal cord dilated anteriorly into a brain, 

 which is enclosed in a bony or cartilaginous brain-case (or skull) ; 

 and a regular muscular heart composed of two chambers. The 

 fishes of these four orders, however, present several important 

 differences amongst themselves. Those of two of them have the 

 bulb of the aorta, or the enlarged part of that vessel which issues 

 immediately from the heart, covered with a strong muscular coat, 

 which, by its contraction, appears to act in some sort as a second 

 ventricle, giving an additional impulse to the blood on its expul- 

 sion from the centre of the circulatory system ; whilst, to prevent 

 the retrogression of the blood under this additional pressure, the 

 inner wall of this part is furnished with numerous valves. The 

 intestine in these Fishes also is provided with a curious spiral 

 valve, attached by one edge to its walls, whilst the other projects 

 freely into the cavity of the intestine ; thus forming a regular 

 spiral canal, through which the alimentary matters are compelled 

 to pass, on their way to the anus. These orders are, I. SELACHII, 



* Omitting the Dipnoi, a small order including the different species of Lepi- 

 dosiren, which we have already referred to as belonging to the Batrachia 

 ($ 534), but which form a sixth group of Fishes according to Professor Miillor. 



