114 Invertebrate Zoology. 



of appendages. They are small unjointed processes lying 

 between the bases of the pair of appendages just considered. 



The seventh pair of cephalothoracic appendages are con- 

 siderably modified and form the opercithim or cover for the 

 abdominal appendages. The two are united in the median 

 line. Certain transverse lines mark their lines of original 

 segmentation. 



The mouth opens in the median line between the bases of 

 the four pincer-bearing appendages. In front of it is a coni- 

 cal elevation, the upper lip. 



The abdomen bears five pairs of modified appendages, each 

 pair resembling the operculum, and covering the large gill- 

 books. By the movement of the abdominal appendages and 

 the gill-books the animals can slowly swim through the water. 



At the base of the caudal spine the opening of the anus 

 will be observed. The external openings of the genital organs 

 will be found on the posterior surface of the operculum. 

 The external openings of the coxal glands have been found 

 in embryos on the basal joints of the fifth pair of cephalo- 

 thoracic legs. 



Internal Anatomy. With a strong knife remove the 

 entire upper portion of the shell of both cephalothorax and 

 abdomen. 



The Muscular System. The strong flexor muscles will 

 be seen to have a longitudinal course along each side of the 

 median plane. The muscles which move the legs are dis- 

 posed laterally to the flexors, and their fibres are arranged, 

 for the greater part, dorso-ventrally. 



The Circulatory System.- -Along the mid-dorsal line, 

 just below the flexor muscles, the elongated heart will be 



