THE LOWER VERTEBRATES 155 



like gill-sacs and unpaired nostrils. They form a small but very 

 interesting class of Vertebrates, undoubtedly very old-fashioned 

 types. Like many other archaic animals, they have a wide 

 geographical position. In the Old Red Sandstone of Caithness 

 there is a dainty little fossil, usually under an inch in length, 

 which seems to have distinct affinities with Cyclostomes, and 

 is in any case " a strange relic of early vertebrate life " in Britain. 

 It is called Pal&ospondylus, but the teacher will of course keep 

 this name, not to speak of Petromyzon and Myxine, as luxuries for 

 his private edification. 



(b) THE LANCELETS 



Occupying a lower rung of the Vertebrate ladder than the 

 Round-mouths are sundry small and dainty creatures known 

 as lancelets Amphioxus, etc. As they are very rare in British 

 waters (though exceedingly common in some seas, such as the 

 Mediterranean), we must not do more than note their general 

 position. They are in 

 many ways simple primi- 

 tive Vertebrates, for in- 

 stance, in having a dorsal FIG. 5?A. Lateral view of lancelet (after Lan- 

 nerve - COrd without a kester) ; N, the tip of the notochord ; C, the cirri 



1t - _ , , . around the mouth ; G, the reproductive organs ; 



Well-defined brain, A> the resp ir a tory atriopore ; An, position of 



having a persistent Un- anus. The zigzag lines represent muscle seg- 



segmented supporting ments - 



axis (notochord) without any trace of skull, and in having very 

 numerous gill-slits without any hint of gills. In other ways the 

 lancelets have diverged on a line of their own. From fishes they 

 are widely removed by the absence of limbs, skull, jaws, brain, 

 eye, ear, and some other important structures. Like other 

 old-fashioned animals, they have a wide geographical distribution, 

 and there are about a score of different kinds. They -mostly occur 

 near the coasts in warm and temperate seas ; they are somewhat 

 sluggish in habit ; they feed on microscopic animals or organic 

 particles which are wafted into the mouth. If a specimen be 

 obtained for the school collection, it is instructive to show that 

 the body-wall is built up of over fifty zigzag dovetailed muscle- 

 blocks like those seen on a fish. 



