292 CHARACTERS OF VERTEBRATE ANIMALS 



name of the group is derived from this peculiarity (Gk. kuklos, 

 a circle ; stoma, a mouth). The skin is devoid of scales, but 

 there are numerous little horny structures lining the mouth- 

 sucker, and a muscular tongue armed with similar bodies can 

 be protruded from the mouth and used as a rasping organ. A 

 single nostril opens on the top of the head, behind this is the 

 eye, and then come seven small round holes, which are the 

 external openings of as many gill-pouches. The egg of the 

 Lamprey develops into a larva which is so unlike the adult that 

 it was formerly thought to be a distinct kind of fish and received 

 the name of Ammocates. 



The Common Hag- Fish (Myxine glutinosa) is shaped like 

 a Lamprey, but the dorsal fin is absent, and the imperfect eyes 

 are covered by the skin. The suctorial mouth is margined by 

 eight barbels. At first sight there appear to be no gill-openings, 

 but there are in reality six pairs of gill-pouches, the outer ends 

 of which are drawn out below the skin into backwardly directed 

 tubes, those on the same side uniting together and having a 

 common opening pretty far back on the under surface of the body. 

 The skin is very glandular, possessing the power of producing 

 vast quantities of slime which sets into a firm jelly. 



PRIMITIVE VERTEBRATES (PROTOCHORDATA) 



We now come to a number of very primitive forms, most 

 of which were classified with the backboneless animals till com- 

 paratively recently. The characters of the backboned animals 

 or Vertebrates have been enumerated and explained in an earlier 

 part of this volume (pp. 60-63), to which reference must be 

 made for details. It may, however, be remarked here that there 

 are three chief characters by which a Vertebrate, or, to use a 

 wider and better term, a Chordate, may be distinguished. 



i. The possession at some period of life of a firm gelatinous 

 rod, the notochord, running longitudinally below the central 

 nervous system. Such a supporting rod can be made out in the 

 embryos of all the forms hitherto described, but in most of them 

 it is sooner or later squeezed out of existence, partly or entirely, 

 by the development of a vertebral column. In some few instances. 



