METAMORPHOSIS OF LAMPREYS. 477 



the body cavity, and pass by two genital or abdominal 

 pores into the urogenital sinus, and thence to the ex- 

 terior. In the male there is an ejaculatory structure, 

 or so-called " penis." There are many more males than 

 females. 



Development of P. planeri. — The ripe ovum has a considerable 

 quantity of yolk, but segmentation is total though slightly unequal. A 

 blastosphere is succeeded by a gastrula. The blastopore persists as the 

 anus of the animal, and there is no neurenteric canal. 



The formation of the central nervous system is peculiar, for the sides 

 of the epiblastic infolding remain in contact instead of forming an open 

 medullary canal. 



In the head region, where the gut is not surrounded by yolk-cells, the 

 mesoblast is formed from hollow folds in " enteroccelic " fashion ; but in 

 the trunk region the cushions of hypoblastic yolk-cells change gradually 

 into mesoblast, and acquire a ccelom cavity in " schizoccelic fashion. 

 Thus the two main ways in which a body cavity arises — (a) from ccelom 

 pouches of the archenteron, (d) from a splitting of solid mesoblast rudi- 

 ments — are here combined. 



Metamorphosis of Lampreys. — The larvre live wallowing in 

 the sand or mud of streams, and feed on minute animals. Those of P. 

 planeri are so unlike the adults that they were once referred to a distinct 

 genus Ammoccetes, and though a Strasburg fisherman, Baldner, is said 

 to have discovered their true nature about two hundred years ago, the 

 fact was overlooked until August Miiller traced the metamorphosis in 

 1856. In the small lampern the change to the adult state is sometimes 

 postponed until the autumn of the fourth or fifth year, when it completes 

 itself rapidly. Less is known about the metamorphosis of the other 

 species. 



In the Ammoccetes, or larva before metamorphosis, the head is small, 

 the dorsal fin is continuous, the upper lip is semicircular, the lower lip 

 is small and separate, the mouth is toothless and not suctorial, the 

 brain is long and narrow, the eyes are half made and hidden beneath 

 the skin ; the future gullet, as distinguished from the respiratory tube, 

 is not yet developed. 



Lampreys are distributed in the rivers and seas of north and south 

 temperate regions. They are often used as food. Besides Petromyzon 

 there are several related genera, e.g. Mordacia and Geotria, from the 

 coasts of Chili and Australia, and Ichthyomyzon, from the west coast 

 of N. America. Certain structures called "conodonts," from very 

 ancient (Silurian) strata, have been interpreted as teeth of lampreys 

 or hags. 



