382 



Vertehrata. 



Fish, which are represented to-day by such numerous genera and 

 species, have also played an important part in earlier periods; 

 the Teleosteans, which preponderate at the present time, arose 

 comparatively late ; whilst the Ganoids, which now include a few 

 species only, were for long very abundant. 



Skeleton entirely cartila- r 



ginous. 

 Homy or cartilaginous rays. 

 Scales absent. 

 Operciilum absent. 

 Swim-bladder absent. 



Synopsis op the Oedees. 



Cyclostomi 



Selachii 



Skeleton of cartilage and 

 bone. 



Bony rays. 



Scales present. 



Operculum present. 



Swim-bladder or lung pre- 

 sent. 



Ganoidei 

 Dipnoi 



Teleostei 



Oonus arteriosus well-de- 

 veloped. 

 Spiral valve in intestine. 

 Miillerian ducts present. 



) Oonus rudimentary. 

 Spiral valve absent. 

 Miillerian ducts absent. 



Order 1. Cyclostomi. 



The Cyclostomes form a small group, differing in many respects 

 from other Pisces. The body is cylindrical, vermiform and apodous ; 

 the skin is naked ; the skeleton is entirely cartilaginous ; the noto- 

 chord is unconstricted ; ribs are absent. There is a complicated oral 

 and branchial skeleton, which can with diflSculty be reduced to the 

 common type of piscine visceral skeleton. There are usually six or 

 seven (in a few, a still larger number) gill-pouches on each side 

 (.see p. 373); the mouth is provided with horny teeth, but 

 true teeth are absent ; the olfactory organ is unpaired ; the caudal 

 extremity straight ; and there is a continuous dorsal fin (c/. in other 

 respects, the account given for Pish in general. 



The Oyclostomes are most nearly allied to tbe Selachians ; their peculiar 

 characters are without doubt to be attributed partly to their peculiar mode of 

 life, as parasites or carrion-feeders. 



1. The Nine-eyes or Lampreys (Petromyzon) have a circular suck- 

 ing mouth with horny teeth; seven small gill-apertures on each side leading 

 into gill-pouches ; which do not open directly into the mouth, but into a short 

 tube ventral to the oesophagus, closed behind but anteriorly in communication 

 with the mouth. Eyes are well developed. The Lampreys attach themiselves by 

 suction to living Fish, which they devour ; they also feed on smaller animals. 

 Three species live in England ; two are marine, but can make their way up 

 into fresh water — P. Marinus, up to Im. long, and the small P. Fluviatilis, 

 (Pricke) ; whilst the third and smallest species (P. Planeri) is exclusively a fresh- 

 water form. Lampreys undergo a metamorphosis; the larvae (J.mmoccEfes), 

 which in P. Planeri may be three or foui- years old and of a considerable size 



