PHYLUM ARTHROPODA 



509 



one or more of the gills in every segment are absent or vestigial, 

 and the following table, or " branchial formula," shows the actual 

 number and arrangement of these organs, ep standing for epipodite, 

 and r for the vestige of a gill. 



By adding up the columns vertically we get the number of gills 

 in each segment ; by adding them horizontally, the number of each 

 kind of gill ; and by adding together the results obtained by either 

 method, the total number of gills, viz., eighteen complete gills 

 with two vestiges and seven epipodites. 



The excretory organs differ both in position and in form 

 from those of Apus. There are no shell-glands, but at the base 

 of each antenna is an organ of a greenish colour, the antennary 

 or green gland, by which the function of renal excretion is per- 

 formed. The gland (Fig. 403) is cushion-shaped, and consists of 

 three parts (1) a central saccule (s.) of a yellowish colour, occupy- 

 ing the mid-dorsal region, and consisting of a sac divided into 

 numerous compartments by partitions, and communicating with 

 (2) the outer or cortical portion (c. p.). of a green colour, consisting 

 of a glandular network formed of anastomosing canals, and com- 

 municating in its turn with (3) a white portion (w. p.), formed of a 

 single tube partly converted into a sponge-work by ingrowths 

 of its walls. The whole organ is lined by glandular epithelium, 

 and the white portion discharges into a thin- walled sac or urinary 

 bladder (bl.) which opens by a duct (d.) on the proximal segment of 

 the antenna. The glands already referred to as occurring in the 

 gills are also supposed to have an excretory function. 



The circulatory organs are in a high state of development. 

 The heart (Figs. 401,404, h.) is situated in the dorsal region of the 

 thorax, and is a roughly polygonal muscular organ pierced by 

 three pairs of apertures or ostia (o.}, guarded by valves which open 

 inwards. It is enclosed in a spacious pericardial sinus (Fig. 404, 

 pc.), which contains blood. From the heart spring a number of 

 narrow tubes, called arteries, which serve to convey the blood to 

 various parts of the body. At the origin of each artery from the 



