PHYLUM ANNELIDA 105 



the cross-section of the earthworm what striking differences from the cross- 

 section of Planaria are apparent? Are the layers of the body wall of the earth- 

 worm the same in general as those of the body wall of the frog? those of the 

 intestine of the two animals? In comparing the cross-sections of Hydra, 

 Planaria, earthworm, and frog one may note the following points: the gastro- 

 vascular cavity of the two former animals corresponds to the cavity of the intes- 

 tine of the two latter; the lining epithelium of the intestine in earthworm and 

 frog is the entoderm; the epidermis, the ectoderm; and all of the tissue in 

 between is mesoderm. 



C. GENERAL SURVEY OF ANNELIDS 



1. Polychaetes. The polychaetes are the group of marine annelids to 

 which Nereis belongs. They are characterized by the possession of parapodia. 

 Examine the various specimens available. Note form and segmentation of the 

 body, development of parapodia, presence and location of gills (slender respiratory 

 processes, often arranged in clumps), degree of development of the head. Many 

 of the polychaetes live in tubes secreted by themselves, and hence the parapodia 

 and head regions are often degenerate. 



2. Leeches. Leeches are common annelids of fresh water, readily distin- 

 guished by the presence of two suckers, one at each end of the body. Examine 

 the specimens. Are parapodia or setae present? 



