114 THE EVOLUTION OF CONTINUITY 



and as these fall into one or other of the two great divisions, 

 Invertebrata and Vertebrata, we shall take as examples 

 the invertebrate earthworm and the vertebrate fish. The 

 case of the Fish, however, we shall leave to a later chapter 

 dealing with the evolutionary factors which probably were 

 at work in producing the ascending scale of living Con- 

 tinuity. In this way undue repetition will be avoided. To 

 both the earthworm and the fish we would attribute a 

 serially-medusoid derivation, but without the inference that 

 the fish has evolved from any primitive invertebrate type 

 possessing characters in any way approaching those of the 

 worm. Our suggestion is that the vertebrates took a line 

 of their own distinct from invertebrate segmental organisms. 



The Earthworm. 



This familiar animal is invertebrate and typically 

 segmental, its segmentation being truly reflected by the 

 succession of annulations on its body surface. In all there 

 are some 150 of these markings, and between any successive 

 two there is a segment. External annulations, however, 

 are not always a true guide to the number of segments in 

 an annelid, as in the case of the Leech, in whom several 

 annulations go to one segment. The mouth is situated at 

 one end of the earthworm and the anus at the other, and 

 the alimentary tract is a continuous tube running down 

 the whole length of the cylindrical body. But between 

 the digestive tube and the body walls there is a space, or 

 cavity, the ccelom, divided by thin partitions into a series 

 of separate chambers, one for each segment ; and the true 

 segment is all that is enclosed by two such partitions. The 

 general plan of the worm's body might therefore be likened 

 to a series of 150 pill-boxes gummed together end to end, 

 with a hollow glass tube passing down centrally through 

 all the partitions in the series. The glass tube would be 

 the alimentary tract, each box represent a segment, and 

 its cavity one of the ccelomic subdivisions. 



The body walls enclosing the ccelom are composed of 

 the following layers from without inwards : the cuticle 

 (which is not cellular, but a hardened secretion) ; the 

 ectoderm, which secretes the cuticle ; a very thin layer 



