224 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



portunity to make observations with young cyclostomes or ganoids, but 

 I am assured by a number of observers who have reared young Pe- 

 tromyzon that they are very sluggish during the early larval stages. 

 From Dr. Mark, who has had a wide experience iu raising the young 

 of Lepidosteus, I learn that the same is true of that species. Of Amia, 

 Reighard (:03) says, " When hatched, the larvae are about 7 milli- 

 metres long, colorless and incapable of progressive movement, and they 

 either attach themselves to the nest material by the adhesive organ at 

 the end of the snout, and remain suspended in a vertical position, or 

 lie on their sides on the bottom. They remain in the nest for about 

 nine days after hatching." Dr. Raymond Pearl of Michigan University, 

 who has carried on extensive physiological experiments on the reactions 

 of larval Amia to stimuli of various kinds, has kindly furnished me with 

 the following data. 



During the first day after hatching the young Amia stay on the 

 bottom of the nest, exhibiting little movement except that of wriggling 

 their tails. The second day they swim an inch or so, but the move- 

 ments are not sufficiently co-ordinated to keep them moving any con- 

 siderable distance. The third day they can swim a little farther, about 

 four inches at one time. They cannot avoid obstacles, and after striking 

 against them show a thigmotactic reaction, and drop to the bottom. 

 The first ready response to optical stimuli comes at about the fifth day. 

 By the sixth or seventh day they begin to swarm, and swim as older 

 fish do. 



In following the development of the optical refl^ex apparatus in Amia, 

 we have seen that this apparatus is in process of development during 

 the first ten days of larval life, and all the anatomical connections are 

 established about the fifth day. This close correspondence of the 

 anatomical development, with the development of activities, is almost 

 conclusive evidence as to the connection of this apparatus with the 

 flight reflex. 



2. Relation of the State of the Apparatus to Activity. 



Another line of evidence from the side of comparative physiology may 

 be drawn from the relative development of the apparatus in species 

 which are either very active or very sluggish. In any group, as, for 

 example, the teleosts, the apparatus has its highest development in those 

 animals which are most active. In the predatory and rapacious bluefish 

 (Pomatomus), the apparatus is highly developed, Reissner's fibre having 

 a diameter of ten micra. In Luphius, which is a much larger fish, but 



