266 OUTLINES OP EVOLUTIONARY BIOLOGY 



though generally complicated by the development of tentacles, 

 skeleton, &c. The development of coelomic pouches (Fig. 13, 

 XI XIII, c.p.) as outgrowths of the primitive digestive cavity, 

 and the conversion of these into mesoblastic somites (Fig. 13, 

 XV, in.8.), arranged serially or metamerically down each side of 

 the body, mark the transition from the unsegmented and 

 coelenterate condition to the metamerically segmented l and 

 co3lomate condition. The cavities of the ccelomic pouches form 

 the coelom or body cavity, which is at first transversely sub- 

 divided into compartments, as it still is in the adult earthworm, 

 while their walls form the third germ-layer or mesoblast, lying 

 between the epiblast which covers the surface of the body and 



FIG. 1 1 8. The Lancelet (Air.phn.xva lar.ceolatus], X 2. (From a photograph.) 



the hypoblast which lines the digestive cavity. Along the mid- 

 dorsal line of the body a strip of epiblast sinks down (Fig. 13, 

 XI, n.pl.) and becomes folded into the form of a tube (Fig. 13, 

 XII XIV, n.t.), the rudiment of the central nervous system 

 (brain and spinal cord), and beneath this tube a long strip of 

 hypoblast becomes nipped off from the roof of the gut, forming 

 the notochord (Fig. 13, XI XIV, not.) or axial skeletal rod 

 the foundation around which in higher types the vertebral 

 column is built up. A little later the front part of the gut 

 becomes pierced by gill- slits for purposes of respiration and 

 the primitive chordate condition is thus fully attained. 



Amphioxus (Fig. 118) does not progress much beyond this 

 stage in its development. It never acquires any limbs but swims 

 about by lateral undulations of the body caused by the contraction 



1 Metameric segmentation has, of course, nothing to do with the segmentation of 

 the ovum, but is the term applied to the transverse division of the body in many 

 animals into a series of joints or segments arranged longitudinally, each of which 

 typically repeats the essential features of all the others. 



