30 



INVERTEBRATE ZOOLOGY 



an anterior and a posterior, and a podobraiich, but on most of 

 the somites a less number is present. 



Exercise 18. Construct in your notebook a table showing the 

 arrangement and number of the gills and also of the epip- 

 odites and their relations to the somites bearing them, 

 as follows : 



Begin with somite 13 and indicate by a " + " under the 

 proper head opposite the number of each somite the presence 

 of the gill or epipodite, and by a " " its absence. 



The internal organs. With strong scissors and forceps care- 

 fully remove the shell from the entire dorsal surface of the ani- 

 mal, taking great care not to disturb the organs lying beneath. 

 Notice just beneath the shell a pigmented membrane. This is 

 the under-skin; it is composed of a layer of connective tissue, 

 gland-cells, nerves, and blood, on the outer surface of which is 

 the layer of epithelial cells called the hypodennis, the matrix of 

 the shell. Entirely remove the under-skin. Study the organs 

 as they lie, without disturbing them. Notice in the cephalo- 

 thorax, first, the large sac-like stomach just back of the rostrum 

 and connected by muscles with the anterior body-wall. On 

 each side of the stomach will be seen the cut ends of a mass of 

 muscle fibres. These are the mandibular muscles. Demonstrate 

 their connection with the mandibles. Just back of the stomach 



