300 M. A. Miiller on the Development of the Lampreys. 



anterior margin of the mouth increases in a regular ratio ; thus, 

 in the Ammocoetes it is 4i, in the metamorphosis 6-7, and in 

 the developed Petromyzon 9 millimeters. The cleft which 

 separates the upper from the lower lip in the Ammoccetes was 

 still distinctly present in some animals, but had completely dis- 

 appeared in others. 



The papillae of the mouth were reduced, but bore no horny 

 armature, and the veil of the mouth still existed in some indi- 

 viduals, especially those which exhibited the cleft between the 

 upper and lower lips most distinctly. When the opening of the 

 mouth was completely rounded, the veil was reduced to a small 

 remnant. The branchial apertures had lost the external valves 

 which in Ammoccetes prevent the ingress of water; and, in the 

 most developed individuals, these apertures were furnished with 

 a border. The inner branchial apertures were narrowed, but 

 wider than in the Lamprey. In the structure of the oesophagus 

 and other particulars of their anatomy, the animals also exhi- 

 bited the same intermediate condition. The ova in the ovaries 

 had already become white and opake ; those present in the 

 ordinary Ammoccetes being transparent. They exhibited the 

 germinal vesicle distinctly, and the testes had developed cells 

 for the formation of spermatozoa. 



The metamorphosis goes on rapidly from this point. In 

 sixteen days the yellow teeth made their appearance in many 

 animals, and the sucking apparatus was in action ; but it had 

 not acquired its ordinary energy in four weeks in animals kept 

 in confinement. 



With this change of form comes a corresponding change in 

 the mode of life. The Ammoccetes shun the light, and bury 

 themselves in the sand at the bottom of the water. Their 

 respiratory organs are protected by the network in the mouth ; 

 they live upon the substances which are collected by it in their 

 mouths, and their oesophagus exhibits ciliary epithelium. The 

 author found the shells of Bacillaria in all the Ammocoetes which 

 he examined. The Petromyzon, on the contrary, being well 

 furnished with eyes, seeks the light, and swims about in the 

 clearest water, or fixes itself by suction, when respiration is 

 effected by the ingress and egress of the water through the 

 external branchial apertures. 



There is therefore no doubt that the Ammoccetes are the larvae 

 of Petromyzon, just as the Tadpoles are the larvae of the Frog. 

 As however only one European species of Ammoccetes has been 

 described, although we have several species of Lampreys, the 

 author thought it worth while to examine the Ammoccetes of the 

 River Lamprey, He found this to be exactly similar to that of 

 the small Lamprey above described ; both possess a gall-bladder 



