54 Laboratory Guide in Zoology 



V. — How many legs has each segment of the thorax ? 



Make a drawing of one of the legs as it appears under 

 the lens, showing distinctly in the drawing the parts of 

 which it is composed. In female specimens, there is a 

 thin membrane or covering — the egg pouch — on the 

 ventral surface of the thorax at the bases of the legs. 



VI. — Study the plate-like gills on the ventral surface of the 

 abdomeii. Why should these creatures be found only 

 in moist situations .' 



THE LOBSTER, OR CRAYFISH 



The lobster is one of the most satisfactory animals to 

 study. It is better to have fresh specimens that have not 

 been boiled, but if these are not easily obtainable, boiled 

 specimens may be substituted. In regions where cray- 

 fishes are abundant, these may be used instead of the 

 lobster. 



EXTERNAL ANATOMY 



I. — Does the lobster show a more marked bilateral sym- 



metry than the earthworm ? Study the hard, strong 

 covering of the body ; this is a true outer skeleton or 

 exoskeleton. Notice the soft, pliable places in the 

 skeleton which permit the bending of the body. These 

 places differ from the rest of the skeleton only in 

 having no calcium. 



II. — Axial Arrangement. In the axial arrangement of 

 parts what resemblances can you find between the 

 earthworm and the lobster.? Both have a ringed or 

 segmented structure, but in the lobster this is shown 

 only in the posterior part of the body. Notice that 

 the body axis is separated into two great divisions. 

 The anterior one includes the head and thorax, closely 

 united, and is called the cephalothorax. The posterior 



