I 9 4 AN INTRODUCTION TO ZOOLOGY 



formed of the entoderm and inner layer of mesoderm. The double 

 transverse walls of the somites become approximated and persist 

 as the septa, and a groove is formed in the ectoderm outside. This 

 forms the intersomitic groove, so that the animal becomes ringed, 

 and the segmentation which was first laid down in the mesodermal 

 bands is visible externally. 



By this time the embryo as a whole has become considerably 

 elongated and distinctly wormlike in appearance, and the subsequent 

 course of its development need not be noticed in detail. The 

 ectoderm gives rise to the epidermis of the adult, which in its turn 

 secretes the cuticle and the setae. At a somewhat earlier period, 

 even before the coelom surrounds the gut, a line of modified cells 

 appears in the ectoderm on each side of the mid ventral line, and this 

 later sinks into the coelom and produces the ventral nerve cord. 

 Immediately lateral to these lines are two other longitudinal rows 

 of special cells, which are similarly passed into the ccelom, and these 

 are the primordia or beginnings of the nephridial tubes. Gonads 

 and genital ducts appear to be derived from "the mesoblast. The 

 somatic mesoderm supplies the muscles and blood vessels of the 

 body wall and the outer layei of coelomic epithelium. The muscles, 

 blood-vessels and covering epithelium of the gut wall are derived 

 from the splanchnic mesoderm. The entoderm forms the main 

 part of the gut lining of the adult, but is supplemented by an anterior 

 invagination of ectoderm, the stomodceum, and a much smaller 

 posterior ectodermal invagination, the proetodoeum, which forms the 

 anus. 



As growth proceeds new somites are added at the posterior 

 end, just in front of the original teloblasts, which therefore mark 

 the growing point of the worm. The first of the mesodermal somites 

 to be laid down is the one surrounding and partly behind the mouth, 

 i.e. the peristomium, and thus we see that while the whole of the 

 remaining parts of the worm is developed in connection with the 

 somites and is metamerically segmented, there remains the small 

 portion in front of the mouth, anterior to the old blastopore, which 

 never forms part of this series. This is the prostomium. The 

 adult worm furnishes a splendid example of serial homology in the 

 metameric repetition of its parts. 



From the free-living coelomate Lumbricus we now pass on to 

 consider another quite different and more degenerate type of worm 

 which is parasitic and only distantly related to Lumbricus. 



Tce/i/a solium, a parasitic flat worm. 



Tceriia solium is the tapeworm that is commonly found 

 living parasitically in the intestine of man in European countries. 



