224 MANUAL OF ZOOLOGY sect. 



divisible into three sets, — first, podobranchiiz 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, pleurobranchioz 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 an ten nary 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 

 bladder which opens by a duct 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. 125, 127, 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. 127, 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 

 end of the heart arise five vessels, and from the posterior 

 end two, which are practically united at their origin. 



All these arteries branch extensively in the various organs 

 they supply, becoming divided into smaller and smaller 

 offshoots, which finally end in microscopic vessels called 

 capillaries. These latter end by open mouths which com- 

 municate with the blood- sinuses (Fig. 128, s), spacious 



