4O2 MANUAL OF ZOOLOGY. 



of the great blood-vessels. The blood itself is colourless. No 

 kidneys have as yet been discovered, and there is no lymphatic 

 system. There is no skeleton properly so called. In place of 

 the vertebral column, and constituting the whole endoskeleton, 

 is the semi-gelatinous cellular notochord, enclosed in a fibrous 

 sheath, and giving off fibrous arches above and below. The 

 notochord is, further, peculiar in this, that it is prolonged quite 

 to the anterior end of the body, whereas in all other Vertebrates 

 it stops short at the pituitary fossa. There is no cranium, and 

 the spinal cord does not expand anteriorly to form a distinct 

 cerebral mass. The brain, however, may be said to be repre- 

 sented, since the anterior portion of the nervous axis gives off 

 nerves to a pair of rudimentary eyes, and another branch to a 

 ciliated pit, believed to represent an olfactory organ. The 

 generative organs (ovaria and testes) are not furnished with 

 any efferent ducts (oviduct or vas deferens). The generative 

 products, therefore, must be admitted into the abdominal 

 cavity, and gain the external medium by the "abdominal 

 pore." 



ORDER II. MARSIPOBRANCHII ( = Cydostoma, Owen ; and 

 Cyclostomata, Miiller). This order includes the Lampreys 

 (Petromyzonidce) and the Hag-fishes (Myxinidcz), and is defined 

 by the following characters : The body is cylindrical, worm- 

 like, and destitute of limbs. The skull is cartilaginous, with- 

 out cranial bones, and having no lower jaw (mandible). The 

 notochord is persistent, and there are either no vertebral centra, 

 or but the most rudimentary traces of them. The heart con- 

 sists of one auricle and one ventricle, but the branchial artery 

 is not furnished with a bulbus arteriosus. The gills are sac- 

 like, and are not ciliated. 



The type of piscine organisation displayed in the Marsipo- 

 branchii is of a very low grade, as indicated chiefly by the 

 persistent notochord without vertebral centra, the absence of 

 any traces of limbs, the absence of a mandible, and the struc- 

 ture of the gills. 



Both the Lampreys (fig. 170, A) and the Hag-fishes are ver- 

 miform, eel-like fishes, which agree in possessing no paired 

 fins to represent the limbs, but in having a median fin running 

 round the hinder extremity of the body. The skeleton re- 

 mains throughout life in a cartilaginous condition, the chorda 

 dorsalis is persistent, and the only traces of bodies of vertebrae 

 are found in hardly perceptible rings of osseous matter de- 

 veloped in the sheath of the notochord. The neural arches of 

 the vertebrae, enclosing the spinal cord, are only represented 

 by cartilaginous prolongations. The mouth in the Hag-fish 



