360 MANUAL OF ZOOLOGY sect. 



in a tough shell, and is discharged when development has 

 only begun. In the rabbit the fertilised ovum is received 

 into the uterus and there undergoes its development, the 

 young rabbit when born differing little, save in size, from the 

 adult. The nourishment of the fee f us or uterine young of 

 the rabbit is effected by means of a special vascular structure 

 known as the placenta, by means of which nutrient material 

 passes from the blood of the mother to that of the foetus ; 

 and after birth the young rabbit receives its nourishment 

 for a time exclusively from the secretion of a set of glands 

 of the mother — the mammary or milk glands. 



CLASS I. CYCLOSTOMI 



The lowest of existing Craniate Vertebrates are certain 

 fish-like animals known as "lampreys" and "hag-fishes," 

 or " slime-fishes," which are looked upon as constituting the 

 class of Craniata, to which the name of Cyclostomi is ap- 

 plied. Of them it is here possible only to make the briefest 

 mention. The lampreys (Petromyzon and other genera) 

 and the hag-fishes or slime-fishes (Myxine and Bdellostoma) 

 are somewhat eel-like in general shape, that is to say, they 

 have a long and narrow body without marked external dis- 

 tinction into regions, and with a soft and slimy integument. 

 Of the fins of such a fish as the dogfish the median or un- 

 paired series alone are represented, paired fins corresponding 

 to the limbs of the higher Craniata being entirely absent. 

 There is a dorsal fin divided into two in the lampreys, undi- 

 vided in the hag-fishes, which is continued as a tail fin round 

 the posterior or caudal extremity of the body. On the 

 lower or ventral surface of the anterior or head-end is a deep 

 hollow — the buccal funnel, much more conspicuous in the 

 lampreys than in the hags, at the bottom of which the small 



