394 THE ANIMAL KINGDOM. 



The organs of the lateral line consist of (i) pyriform sense-cells terminated at 

 their outer ends by a single sense-hair, at their inner by a nerve-fibre ; (2) of sup- 

 porting cells either surrounding or mixed with (i). The sense-cells are shorter 

 than the supporting-cells, and in this point nerve-eminences differ from the terminal 

 buds of the skin of Teleostei where the two kinds of cells are of equal length. The 

 nerve-eminences are circular, oval, or ridge-like, and the ridges may be almost con- 

 tinuous with one another. They are either freely exposed, and then their surface 

 is protected by a delicate cuticle perforated by the sense-hairs : or they are sur- 

 rounded by a delicate hyaline tube as in Amphibia and some adult Teleostei: or 

 they are sunk in furrows or canals which have apertures leading upwards from spot 

 to spot, as in some Pisces. When thus sunk, the dermal skeleton generally comes 

 into relation with them. In the region of the head they are supplied by the fifth 

 nerve (and by the seventh in the Skate) : on the body by a branch of the vagus, the 

 lateral line nerve. 



They appear to be segmentally arranged in the embryo in many instances, and 

 it has been recently pointed out that a single eminence is seated at the dorsal end 

 of each branchial cleft, and of the mouth at an early period. Hence the name 

 'branchial sense-organ' applied to them in this region. It is possible that the 

 olfactory mucous membrane is derived from such an organ. The eminences mul- 

 tiply by division. In a few instances they are found scattered over the whole body, 

 e.g. Mugil (see p. 85), a condition perhaps to be regarded as primitive. 



Nerve-eminences, &c., Merkel, Endigungen der Sensibeln Nerven in der Haut 

 der Wirbelthiere, Rostock, 1880; Leydig, Festschrift zur Feier des loo-jahrigen 

 Bestehens der Natf. Gesellsch. in Halle, 1879. Branchial sense-organs, Beard, 

 Q. J. M. xxvi. 



On the origin of azygos fins, see p. 101, under interspinal bones. 



CLASS AMPHIBIA. 



Ichthyopsida with an integument rich in glands or glandular cells and 

 devoid of an epidermic exo-skeleton. The azygos system of fins is always present 

 in the larva and in the adult of certain sub-groups, and is never supported 

 by fin-rays. The limbs are borne by well-developed shoulder- and pelvic- 

 girdles, and the latter is connected to a sacral vertebra : they consist as con- 

 trasted with the limbs of Pisces of an upper and lotver limb, with a hand 

 and foot, thus agreeing with those of higher Vertebrata. The alimentary 

 canal always terminates with a cloaca common to the rectum and tirogenital 

 ducts, to which is appended ventrally an allantoid bladder. 



The epidermis in the young larva is ciliated. In older larvae it con- 

 sists of only two layers of cells, an outer with a striated cuticular border, 

 and an inner, in which mucous cells ( = Leydig cells) are to be found 

 scattered. In the adult the outer layer is cornified and is cast off period- 

 ically in the Anura and the Urodela ; but the inner layer forms a thick 

 stratum mucosum, and certain of its cells are enlarged and flask-shaped, and 

 are supposed to yield a secretion which facilitates the moult of the cornified 

 layer. Some of the cells contain pigment. The surface of the epidermis 



