222 HIGH SCHOOL ZOOLOGY. 



as through the rest of the body, is assured by heai-t-like loops 

 which connect the principal longitudinal vessels. These are 

 situated above and below the tubular intestine, and above and 

 below and on each side of the nerve-cord. 



6. The Discophora differ from the other Annelids in the entire 

 absence of bristles, and in the presence of a sucker immediately 

 below the posterior aperture of the intestine. Most forms also 

 have the segment in front of the mouth (prestomium) converted 

 into a sucker. In accordance with their habits of life, we find 

 conspicuous structural differences in the Leeches, as compared 

 with the other Annelids. They are sometimes temporary para- 

 sites, like the medicinal leech in Europe (Hirudo) or its Ameri- 

 can representative Macrohdella, living upon the juices of other 

 animals, which they suck and store up in a sacculated intess 

 tine, or carnivorous forms with a simpler intestine preying on 

 the smaller Invertebrates (N'ephelis). Both of these groups 

 have jaws, which. "are used either for inflicting a wound be- 

 fore sucking, or for comminuting their food. Other para 

 sitic forms are destitute of jaws, but have instead a protractile 

 proboscis. Common examples are the fish-leech (Piscicola or 

 Ichthyohdella), and the various species of Clepsine, large form- 

 of which attack the pond-turtles, while smaller species prey on 

 the pond-snails. Another interesting form, destitute of the 

 anterior sucker, is the curious little Branchiobdella, several 

 species of which are to be found on the gills, and on the head- 

 parts, of the various species of crayfishes. 



7. In contrast to the Annelids, the lower Classes of Vermes do 

 not possess metameric segmentation. Certain marine forms — 

 Gephyrea — have segmental organs, but the coelom is undivided 

 and the r-Drve-cord alike throughout. In the other Classes 

 (Rotifera, Nematelminthes, and Plathelminthes), to which, for 

 the most part, unfamiliar and inconspicuous forms belong, a 



