II.] THE CRAYFISH AND LOBSTER. 205 



a. The relations between any two of them are identical 

 with those described above, motion being possible 

 in only one plane. 



b. The individual segments are incapable of rotation 

 upon each other. The limb as a whole can be 

 rotated; owing to the variation in the angles of 

 inclination of its several joints. 



4. Make a median longitudinal section of the whole 

 body, similar to that described in Sect. B. 5 ; pick 

 away the viscera under water, wash and examine 



a. The flexor abdominis muscle; a powerful mass, ex- 

 tending from the anterior border of the thoracic 

 epimeron and endophragmal system to the base of 

 the telson. It arises by a series of slips, which 

 wind round each other in a highly characteristic 

 manner and are inserted into the abdominal sterna. 



b. The extensor abdominis; a much less powerful 

 muscle, arising immediately above a. Its fasciculi 

 are inserted into the abdominal terga. 



c. The levator abdominis ; arising, immediately above 

 the extensor, from the antero-dorsal border of the 

 thoracic epimeron. It passes obliquely backwards, 

 to be inserted, laterally, into the first abdominal 

 segment. 



This muscle is continuous with a series of fibres 

 which skirt the entire anterior border of the thoracic 

 epimeron and attach it to the cervical groove. 



Note that the parts of the axis are, like those of 

 the appendages, capable of motion only in one plane. 



d. The adductor muscle of the mandible ; a powerful 

 fan-shaped mass, passing obliquely upwards from 



