THE FISHES 157 



sand in warm seas, the nine species known being found in 

 as many different regions. A lancelet may be regarded as 

 a vertebrate reduced to its lowest terms. Instead of a 

 jointed back-bone, it has a cartilaginous notochord, running 

 from the head to the tail. A nervous cord lies above it, 

 enclosed in a membranous sheath. No skull is present, and 

 the nerve-cord does not swell into a brain. There are no 

 eyes and no scales. The mouth is a vertical slit, without 

 jaws. There is no trace of the shoulder-girdle (shoulder- 

 blade and collar-bone) or pelvis (hip-bone) from which 



FIG. 98. The California lancelet (Bramhiostoma californiense). Twice the natural 

 size, g, gills ; I, liver ; m, mouth ; n, nerve-cord ; nc, notochord. 



spring the paired fins, which, in true fishes, correspond to 

 arms and legs. The circulatory system is fish -like, but there 

 is no heart, the blood being driven about by the contraction 

 of the walls of the vessels. Along the edge of the back and 

 tail is a rudimentary fin, made of fin-rays connected by mem- 

 brane. In the character and arrangement of its organs the 

 lancelet is certainly like a fish, but in degree of develop- 

 ment it differs more from the lowest fish than the fish does 

 from a mammal. 



150. Lampreys (or Cyclostomes). The class of lampreys 

 stands next in development (Fig. 99). The notochord gives 

 way anteriorly to a cartilaginous skull, in which is con- 

 tained the brain, of the ordinary fish type. There are eyes, 

 and the heart 'is developed, and consists of an auricle and 

 a ventricle. As distinguished from the true fish, the lam- 

 preys show no trace whatever of limbs or of the bones 

 which would support them. The lower jaw is wholly want- 

 ing, the mouth being a roundish sucking disk. The fins 



