GAMMARUS. 347 



muscle fibres surrounding them during systole, and 

 through them the blood from the pericardial cavity enters 

 the heart. Its pulsations may be observed with the naked 

 eye, and range from 120 to 130 beats per minute. 



The pericardial cavity (PI. III., fig. 1, p.c.) extends 

 throughout the whole length of the body, from the first 

 segment of the thorax to the last segment of the abdomeu, 

 which it occupies almost entirely. In section (PI. IV., 

 fig. 1) it is crescent-shaped, its dorsal wall lying close 

 below the integument, which it follows closely down to 

 the junction of the terga and the epipodal plates. The 

 arched ventral wall (PI. IV., fig. 1, p.) is separated from 

 the alimentary canal and its diverticula by connective 

 tissue, and is closely applied to the ventral wall of the 

 heart. It is perfectly closed, except where the efferent 

 vessels (PI. III., fig. 1, eff. v.) from the appendages open 

 into it, above the level of the heart. In addition to the 

 heart, it encloses the posterior aorta, and the beginning 

 of the anterior aorta. 



Four arteries arise from the heart. These are the 

 median dorsal posterior aorta, the median dorsal anterior 

 aorta, and the paired facial arteries, the last three 

 arising together at the beginning of the first thoracic 

 segment (PI. II.; PI. III., fig. 1). 



The posterior aorta (PI. III., fig. 1, a. post.) arises, in 

 the middle of the sixth segment, from the ventral wall of 

 the heart. It is separated from the heart by a two-lipped 

 valve, hing in a horizontal plane. From the begiuning 

 of the last thoracic segment to the end of the fourth 

 abdominal one, it encloses the two posterior diverticula 

 of the gut. The latter do not, however, lie in its lumen, 

 but are surrounded by a double wall, the outer one corre- 

 sponding to the wall of the aorta, which has apparently 

 been pushed in by the development of the diverticula, so 



