STRUCTURE OF THE LAMPREY. 40S 



size of the ova the ovary is very conspicuous in full-grown 

 forms. When the ova are freed from the ovary they pass into 

 the body-cavity. Each has an oval horny case, with knobbed 

 processes at each end. By these they become entangled 

 together. There are no genital ducts, and the expulsion of 

 the products requires to be investigated. The development 

 is still unknown. 



Besides Myxine glutinosa, two other species are known, 

 one from Japan, another from the Magellan Straits. The 

 genus Bdellostoma, from Southern Seas (off the Cape of 

 Good Hope, etc.), is nearly allied ; it has six or more gill- 

 pouches which open apart from one another. 



Second Type of Cyclostomata. 

 Petromyzon — The Lamprey. 



There are three British lampreys, the sea lamprey {Petro- 

 myzon marinus) over three feet in length, the river lampern 

 {P. fluviatilii) nearly two feet long, and the small lampern, 

 or " stone-grig " (P. branchialis or planeri). They eat 

 worms, small crustaceans, insect larvae, dead animals, etc., 

 but they also fix themselves to living fishes and scrape holes 

 in their skin. As their names suggest, they also attach 

 themselves firmly to stones, and some draw these together 

 into nests. 



The spawning takes place in spring, usually far up rivers. 

 Some observers say that the adults die soon after reproduc- 

 tion. The young are in many ways unlike the parents, and 

 pass through a metamorphosis. To the larvae before meta- 

 morphosis the old name Ammocxtes is often applied. 



Form. — The body is eel-like. There are two unpaired 

 dorsal fins, and another round the tail. Two ridges, one on 

 each side of the anus, Dohrn compares to rudimentary pelvic 

 fins. Otherwise, there is no trace of limbs. 



The Skin is scaleless, slimy, and pigmented. Its structure, 

 like that of Myxine, is complex. Sensory structures occur 

 on the head and along the sides. 



Muscular System. — ^The muscle-segments are well-marked. 

 The suctorial mouth and the rasping tongue are very 

 muscular. 



