222 MANUAL OF ZOOLOGY SECT. 



externally by the gill-cover or pleural region of the carapace 

 (kd.). Each gill consists of a stem giving off numerous 

 branchial filaments, so that the whole organ is plume-like. 

 The filaments are hollow, and communicate with two 

 parallel canals in the stem an external, the afferent 

 branchial vein, and an internal, the efferent branchial 

 vein. 



According to their point of origin, the gills (Fig. 117) are 

 divisible into three sets first, podobranchice or foot-gills, 

 springing from the epipodites of the thoracic appendages, 

 from which they are only partially separable ; secondly, 

 arthrobranchice or joint-gills, springing from the articular 

 membranes connecting the thoracic appendages with the 

 trunk ; and thirdly, pkurobranchice, or wall-gills, springing 

 from the lateral walls of the thorax, above the attachment of 

 the appendages. 



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 performed. The gland is cushion- 

 shaped : it discharges into a thin-walled sac or urinary 

 If/adder, which opens by a duct on the proximal segment of 

 the antenna. 



The circulatory organs are in a high state of development. 

 The heart (Figs. 116, 118, 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 (0.), guarded by 

 valves which open inwards. It is enclosed in a spacious 

 pericardia! sinus (Fig. 118, 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 heart are 

 valves, which allow of the flow of the blood in one direction 

 only, viz. from the heart to the artery. From the anterior 



