BY J. DOUGLAS OGILBY. 395 



less specialised. Doi'sal fin more or less deeply divided by a 

 notch, the posterior portion usually continuous with the caudal. 

 Intestine with a rudimentary spiral valve. Eggs small, fertilised 

 after extrusion. Sexes separate. 



Etymology : — v-rrepma, palate; apnos, entire : in reference to 

 the non-perforation of the palate by the nasal duct. 



Distribution : — Seas and rivers of the temperate zones of 

 both hemispheres. 



All the Lampreys are subject to a metamorj)hosis; during the 

 earlier stage of their existence, when they are known as 

 ammocoetes, the eyes are in a rudimentary condition and they are 

 entirely without teeth, their food consisting solely of vegetable 

 substances gathered from the mud in which they live. 



These ammocoetes are not unfrequently found of an equal or 

 even larger size than individuals of the same species in which the 

 eyes and teeth have already undergone development, this being 

 due to arrested growth of these organs on the pai't of the 

 individual. 



Several distinct genera, such as Ammocmtes, Scolecosovia, &c., 

 have been constituted for the inclusion of these immature forms. 



The suctorial disk which is so characteristic of the Lampreys is 

 useful to them in various ways; it serves as an instrument by 

 means of which they are able to adhere to rocks, piles, sunken 

 logs and the like, and so resist the force of the current and escape 

 the necessity for such continuous and violent muscular exertion 

 as would be imperative in an animal possessed of such feeble 

 swimming powers; by it they are able during the spawning 

 season to remove stones and similar obstructions from that portion 

 of the river bed which has been selected as suitable to the for- 

 mation of the nesting-place or " redd," and, after the task of 

 depositing the ova has been completed, to replace the stones, and 

 so minimise the danger to which the eggs would be exposed in 

 the event of the occurrence of heavy floods during the period of 

 incubation; and finally, by it they are enabled to attach them- 

 selves to the substances which form their food. 



