2l8 HIGH SCHOOL ZOOLOGY. 



CHAPTER VIIL 

 The Vermes and Mollusca. 



1. In last chapter, reference was made to the prevailing heter- 

 onomy of segmentation of the Arthropods, but tlie class of the 

 Myriapods, and especially the genus Peripatus, were described 

 as possessing a more worm-like form and homonomous segment- 

 ation. The latter genus, indeed, is destitute of the jointed 

 appendages almost universal in the Arthropods, and its locomo- 

 tive organs rather suggest the unjointed stumps of the highest 

 worms. These belong to the class Annelida (of the sub-kingdom 

 Vermes), a class which is chiefly represented by marine forms, 

 but of which the earthworm and leech may be selected as more 

 accessible types. 



2. The Vermes admit of no such sharp definition as do the 

 Arthropods, for although bilateral symmetry is present in all, 

 yet some forms are segmented, others unsegmented, and tluis, 

 the structui-e of the body may be extremely different in the 

 various classes. The highest class, the Annelida, includes all 

 the segmented forms, and those, consequently, nearest the 

 Arthropods : a comparison, however, of an earthworm and a 

 leech, on the one hand, with a crayfish on the other, will disclose 

 the essential differences which exist between them. 



3. One of the most notable of these is the absence of any 

 exoskeleton such as that of the Arthropods; a thin cuticle 

 containing chitin represents it, and is formed by underlyino- 

 epidermal (so-called hypodermal) cells, but many of these are 



