CYCLOSTOMES. 221 



in the myxinoids, opens into the mouth. The ears are remark- 

 able in the absence of the horizontal (external) semicircular 

 canal, while in the myxinoids but a single canal is present, 

 which, as it bears an ampulla at either end, may be regarded 

 as representing the anterior and posterior canals of the normal 

 ear (p. 71). 



The vertebral column consists of a large persistent noto- 

 chord surrounded by a fibrous sheath and a membranous neural 

 tube, in which {Petromyzoti) cartilaginous neural arches occur. 

 The cranium is cartilaginous, but is more or less incomplete 

 above, and roofed by membrane. Labial cartilages and carti- 



G 



FlG. 225. Cranium and branchial basket of Pctromyzon, after W. K. Parker. 

 E, otic capsule ; B, branchial basket ; G, gill clefts ; N, nasal capsule ; NT t 

 notochord. 



lages for the tongue occur ; but all traces of jaws pterygoquad- 

 rates, Meckel's cartilage are lacking. The branchial region 

 in the petromyzontes is supported by a complicated cartilagi- 

 nous framework, the branchial basket ; but it is as yet impossible 

 to homologize this with the visceral arches of the higher verte- 

 brates. In the myxinoids the basket is rudimentary. 



The gill slits are tubular, and the folded gills are borne on 

 the walls of pouch-like enlargements of these tubes. The heart 

 lacks a conus arteriosus, and no renal portal circulation occurs. 

 The excretory organs are elongate bands. In the lampreys and 

 Myxine the pronephros is lost in the adult ; but in Bdellostoma 

 it retains its function throughout life, its nephrostomes open- 

 ing into the pericardium. 1 The gonads are unpaired, and in the 

 myxinoids are protandric hermaphrodite in character (i.e., the 

 animal is at one time male at another female), the anterior por- 



1 According to Price, the whole excretory organs of Bdellostoma are pronephric in 

 character. 



