FIXED-GILLED CHONDROPTKRYGIANS. 255 



altogether membranous, lateral dentations of the tongue strong, and 

 arranged in two rows on each side, so that the jaws of these fishes 

 seem to be lateral like those of insects or the Nereides, which induced 

 Linnaeus to place them in the class of Vermes ; the rest of their 

 organization, however, is analogous to that of the lampreys :* the 

 tongue also acts like a piston, and the spine of the back is in the form 

 of a cord. The mouth is circular and surrounded with eight cirri; in 

 its upper margin is a sph'acle which communicates with its interior. 

 The body is cylindrical, and furnished behind with a fin that 

 surrounds the tail. The intestine is simple and straight, but wide 

 and plaited internally ; the liver bilobate. There are no vestiges of 

 eyes. The eggs become large. These singular animals pour out 

 such an abundance of mucus through the pores of their lateral line, 

 that the water of the vases in which they are kept seems to be con- 

 verted into a jelly. They attack and pierce other fishes like the lam- 

 preys. 



They are subdivided according to the external orifices of their 

 branchiae. In 



Heptatremus, Dumer. 

 There are still seven holes on each side, as in the lampreys. 



But a single species is known, Gastrobranche domhey, Lacep., 

 I, xxiii, 1; Petromyzon cirrhatus, Forster; Bl., Schn., p. 532; 

 from the South Seas.f 



Gastrobranchus, Bl. 



The intervals of the branchiae, instead of having separate issues, 

 communicate with a common canal on each side, each of which ter- 

 minates in a distinct hole situated under the heart, near the first third 

 of the whole length. 



But a single species is known, My .vine ylutinosa, L. ; Gastro- 

 branchus c<ecus, BL, 413; the (Glutinous Hag.) From the Arctic 

 Ocean. 



Ammoccetes, Dumer. 



All the parts which should constitute the skeleton, so soft and 

 membranous that they are hardly entitled to the appellation of bone. 

 The general form of these fishes, and external orifices of the branchia? 

 are similar to those of the lampreys, but their fleshy lip is cemicir- 

 cular, and only covers the top of the mouth, consequently they 

 cannot attach themselves to bodies like a true lamprey. They have 

 no teeth, but the opening of their mouth is furnished with a row of 

 small branched cirri. They have no particular trachea, and their 

 branchiae are supplied with water from the oesophagus as usual. 

 Their dorsals are united with each other and with the caudal, form- 

 ing a low and sinuous fold. They inhabit the ooze of brooks, and 



* See the Memoir of Abildgaart, Trans. Soc. Nat. Berlin, vol, X, p. 193. 

 f See the Memoir of SirEv. Home, PhiJ. Trans., 1815. 



