168 ARTHROPODA 



PORCELLIO OR ONISCUS. (Sow-Bug.) 



These animals occur in damp places, such as under stones, 

 logs, etc., and in cellars. They live for the most part on decaying 

 vegetable matter. To what class of the Arthropoda do they be- 

 long? 



1. Notice the shape. Is this an adaptation? 



2. Is the body divisible into head, thorax, and abdomen? 

 Count the number of segments. Is there any evidence of fusion 

 at the posterior end of the body? 



3. Examine the appendages. 



(a) Are the eyes stalked or sessile? 



(6) Only one pair of antennae is present, the first pair being 

 rudimentary. 



(c) The mouth appendages are small. They consist of man- 

 dibles, two pairs of maxillce, and one pair of maxillipeds. 



(d) How many walking legs are there ? Are these all alike ? 



(e) Notice the character and number of the abdominal appen- 

 dages. On the posterior surface of all but the last pair, which 

 are modified to form anal feelers, are gills. These are the only 

 respiratory organs. Why must these animals live in damp places ? 



Make a drawing of the animal from the ventral side. 



CAPRELLA. 



These animals are very common on hydroids, but because of 

 their peculiar shape and slow motions are rather inconspicuous. 

 Watch the animals and see how they move. Is the body kept 

 at rest and moved by the action of the appendages, or how is 

 movement from place to place effected? Are the appendages 

 adapted for grasping? Why are they arranged at the two ends 

 of the body? Watch specimens and see if you can determine 

 on what they feed. 



The form is of interest because of its extreme modification to 

 suit it to the needs of its life. There is some difference in the 

 structure of the male and female. 



1. Count the segments of the body. Do they differ in num- 

 ber and shape in male and female? The first represents the head 



