100 LOWER VERTEBRATES. 



sometimes fails, for the Physoelists in very early life have a communication between 

 the bladder and intestine, and siieh communication is liable to be persistent in a few 

 adults. For this very reason, however, the character in question becomes important 

 for another purpose, — to determine the order of succession of the respective tyjjes. 

 Inasmuch as the Ganoids had and have a similar connection between the air-bladder 

 and intestine, it might reasonably be assumed that the j)hysostomous fishes are nearer 

 the progenitors of the Teleostean stock than the physoclistous ones. Such assump- 

 tion, moreover, has been verified by a consideration of all other parts, and it is now 

 generally admitted that next after the Ganoids should come Physostomes ; but which 

 special group or sub-division it should be is a question more difficult to answer. As 

 already indicated, we shall commence with the eels, but without pretending that they 

 are the most generalized. We must, on tbe contraiy, admit that in some respects they 

 are much more specialized than those which are to follow. It is therefore simply 

 and purely from motives of convenience, and to render future breaks less numerous 

 and abrupt than they would otherwise be, that we adopt the sequence of forms here to 

 be followed. Modified as they are, they nevertheless retain more of the evidences of 

 genetic relations to primitive types than do the highly specialized forms into which 

 the typical fishes grade and culminate. 



Oedee I. — SYMBRANCHIA. 



Before proceeding to the consideration of the typical eels, it will be well to notice 

 a few fishes which are eel-like in habit, and even still more elongated than the common 

 eels, but which in other respects are more like the general stock of fishes. The forms 

 exemplifying this group, which has been called Symbranchia, have been ranged by 

 Professor Cope under two orders, the Ichthyocephali and Holostomi. Both agree in 

 the development of special intermaxillary bones and the position of the supramaxillary 

 behind and parallel to them, but disagree in the development of the shoulder girdle, 

 some (Monopteridfe) having the girdle directly connected with the skull, while others 

 (Amphipnoidfe) have no osseous junction. The palatine bones are well developed 

 and even very broad, being thus quite unlike those of the true eels. Although the 

 species are few, they exhibit such differences of structure as to demand the recognition 

 of four families, the MoNOPTEEiDiE, Symbeaxchid^, Amphipxoid^, and Chilo- 

 BEANCHiD^. The first three are remarkable for an excessive elongation of the abdomi- 

 nal portion, and the approximation of the anus to the tail end rather than the head. 

 The Monopterus Javanejisis has as many as one hundred abdominal and eighty-eight 

 caudal vertebriB ; on the other hand, the Chilobranchidae (a family of doubtful rela- 

 tionship) have only about twenty-one abdominal and fifty-two caudal vertebrae. The 

 monotypic Monopteridaa and Amphipnoidas inhabit the East Indian seas ; the Sym- 

 branchidffi are, partially at least, fresh-water fishes, of which two species are East 

 Indian and one South American. The two known species of the Chilobranchidae are 

 Australasian. The Monopteridse and Amphipnoidas especially exhibit interesting 

 modifications of their branchial and respiratory apparatus. 



Oedee IL — APODES. 



The common fresh-water and sea eels are the typical representatives of an order 

 which has been called Apodes, in that they are the best-known examples of the group, 



