250 Notes, iv. 13. 



forwards, so as to become jugular, they are as a rule, if not invariably, reduced in size ; 

 and they are also, as a rule, modified in such a way as to serve new purposes, to 

 act for instance as instruments of touch. Cf. Ann. d. Sci. Nat., 1872, t. xvi. p. 93. 



19. Cf. Note 17. 



20. Namely, in the latter part of the second book and beginning of the third book. 



21. De Resp. ch. 4. 



22. A. says, minister "as it were" to expiration ; for expiration is limited by him to 

 the expulsion of air from a lung after inspiration. The expulsion of water through gills 

 is analogous to this, but not the same thing. Cf. ii. 16, Note 8. 



23. In these cartilaginous fishes there is no gill-cover ; the gills being placed in a 

 series of distinct sacs or pouches, each of which has its own separate slit-like aperture, 

 which is closed during inhalation by its own muscular sphincter. Compare what is said 

 about the epiglottis (iii. 3, 14 ; ii. 13, Note 5). 



24. In the Elasmobranchii or cartilaginous fishes there are five, and in osseous fishes 

 four, gills on either side, as a rule. But the number is subject to some variations. For 

 instance in Lophius, Cotylis, Diodon, there are but three pairs ; and in Cuchia only two, 

 one being almost rudimentary. On the other hand in some fishes there is a kind of 

 accessory gill, as in Sturgeon, Chimaera ; and in many there is a small vascular apparatus, 

 something like a gill, though without its function, at the top of the respiratory chamber. 

 Each gill consists as a rule of a double row of leaflets. But it is by no means uncommon 

 for the last, that is the fourth, gill in an osseous fish to be, as A. says, furnished with only 

 a single row, e.g. Scarus, Scorpsena, Cottus, most Labroids, etc. So far then the statement 

 in the text is perfectly intelligible. But A. also speaks of the other gills, besides this 

 last one, as being sometimes uniserial. The eel for instance has, he says {^H. A. ii. 13, 8), 

 four uniserial gills ; as also the sturgeon {iWoip) and sundry other fishes. The explana- 

 tion of this statement is, I imagine, as follows. In some fishes the two series of leaflets 

 that constitute a single gill are quite distinct and free from each other throughout. 

 But in most they are united together at their basal ends for a smaller or larger proportion 

 of their depth. In the eel, for instance, this fusion extends one-third of the whole 

 depth ; in the sturgeon two-thirds or more of the whole depth ; in the salmon one- 

 half, and so on. When the opposite rows of leaflets are thus partially united for a 

 considerable proportion of their depth, so as to form a single bifurcated plate, they are 

 considered by A. to form only a single row or an uniserial gill ; just as he considers 

 the two lungs to be really one bifid organ, because they coalesce into a single trachea. 

 Cf. iii. 6, Note i. 



25. Cf. If. A. ii. 13, 7—9. 



26. Because the hotter an animal is, the more perfect must be the arrangements for its 

 refrigeration. 



27. Why can eels and some other fishes live so long out of water? It is usually said 

 that it is owing to the small size of their branchial apertures, which hinders the drying 

 up of their gills. But Bert has shown that this is not the true cause ; the real 

 explanation being that these fishes produce, in a given time, ounce for ounce of body 

 weight, less carbonic acid and therefore require less oxygen than other species. 



28. Seeing that dolphins abound in the Mediterranean, and that the main points in 

 their structure, and their habits of life, are accurately enough described by Aristotle, it 

 seems to me quite impossible either that he can have imagined their mouth to be 

 underneath their body, or, as has been suggested, confounded them with the larger 

 sharks. I agree therefore with Frantzius that the word dolphins in the text is an inter- 

 polation ; and this notwithstanding the objection to that view taken by Meyer, namely, 



