6o Laboratory Guide in Zoology 



The fact that the digestive canal passes at its ante- 

 rior end through a ring of the nervous system is an 

 important characteristic of animals below the verte- 

 brates, or back-boned animals. Find, if you can, nerves 

 passing from the forward ganglion to the eyes, antennae, 

 and antennules. Make a sketch of the nerve system. 



XV. — Excretory Organs. Directly under the antennae, see 

 the green bodies, the excretory glands, whose openings 

 are found in the basal segments of the antennae. 



XVI. — Compare the structural plan of the lobster with that 

 of the earthworm, remembering that both are analogous 

 to a tube within a tube, the edges of both tubes being 

 united at both ends. With this structure in mind, tell 

 which tube the mouth and the anal opening are con- 

 nected with. In which tube do the reproductive and 

 excretory organs lie .' Where do their ducts lead ? In 

 which tube does the liver lie, and into which do its 

 ducts pass .' What is the relation of the skeleton and 

 body muscles to the system of tubes .' 



THE COMMOW CRAB 



Crabs are familiar to all who have wandered along the 

 sea-shore at low tide. They run sideways over the exposed 

 rocks, or crawl leisurely along the bottoms of the pools. 

 They are closely related to the lobster, although very dif- 

 ferent in appearance. 



I. — Study the body as to its general shape. What modifi- 

 cations in the size, shape, and arrangement of the 

 cephalothorax and abdomen make it appear so different 

 from the lobster ? Where is the abdomen situated ? 

 Draw the view presented as you look down upon the 

 back. 



