36 INVERTEBRATE ZOOLOGY 
an anterior and a posterior, and a podobranch, — 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 velations to the somites bearing them, 
as follows: 
No. OF 
Popo. ANT. ARTH. |POST. ARTH. PLEU. EPIP. TOTAL. 
SOMITES. 
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 hypodermis, 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 
