14 Proceediniia, 



by no means possible to draw a hard-and-fast line between 

 the Vertebrates and the Invertebrates, this creature has until 

 quite recently been classed with the Tunicates, which were at 

 one time regarded as a division of the Mollusca, or shell-fish, 

 though these are without shells proper, but are provided with 

 a tough, leathery Idud of covering, a cloak or tunic, whence 

 their name (Tunica — a cloak). And it is indeed difficult to 

 say whether both should not be put back out of the back- 

 boned division, as they differ so strongly from all other Ver- 

 tebrates. 



In the next division, the " Marsipobranchii," to which the 

 Lampreys belong, there is a distinct skull at the end of the 

 notochord, and this contains a small brain, that is, a knob of 

 nerve which is divided into certain definite parts. The heart 

 is an ordinary fibli's heart of two chambers, one, the auricle, 

 — which receives the blood from the body, and the other, 

 the ventricle, a pump which forces it through the body again 

 and so keeps it circulating. The breathing-apparatus, or 

 gills, consists of a tube leading from the mouth and passing 

 tln-ough pouches opening by seven holes on each side of the 

 neck, and out of these the water pours, passing on its way 

 the blood-vessels which line the skin of these pouches. These 

 fishes feed on other fish, such as the cod. One kind, called 

 the Hag, gets into the gills of the fish, and literally eats itself 

 out of house and home, unless we choose to regard the skin 

 and bones as its roof and rafters, for these are all that is left 

 of its unfortunate host. In the Lamprey there are no indi- 

 cations of limbs, and no jaws ; but they have a round mouth 

 fringed with tentacles, and well studded on the inside with 

 horny teeth, with which they rasp away the flesh of their prey. 



The next division includes the Sharks and Eays, and among 

 these we meet for the first time with a regular jointed back- 

 bone, though others of the same division have got no farther 

 than the notochord stage. In the Rays the spinal column is 

 a very good, bony one, and their skin is covered by a mass of 

 scattered, bony points, rendering it a hard matter to cut 

 through it ; and when these points are set closely together 

 their skin is known as shagreen, and is or used to be made 



