276 PHYLUM ARTHROPODA. 



superior abdominal, running along the dorsal surface, and a 

 sternal which goes vertically through the body. This 

 sternal passes between the connectives joining the fourth and 

 fifth ventral ganglia, and then divides into an anterior and 

 posterior abdominal branch. All these arteries are con- 

 tinued into capillaries. 



From the tissues the venous blood is gathered up in 

 channels, which are not sufficiently defined to be called veins. 

 It is collected in a ventral venous sinus, and passes into the 

 gills. Thence purified by exposure on the water- washed 

 surfaces, it returns by six vessels on each side to the peri- 

 cardium. From this it enters the heart by six large and 

 several smaller apertures, which admit of entrance but not 

 of exit. 



The blood contains amoeboid cells, and the fluid or 

 plasma includes a respiratory pigment, hremocyanin (bluish 

 when oxidised, colourless when deoxidised), and a lipochrome 

 pigment, called tetronerythrin. Both of these are common 

 in other Crustaceans. 



Respiratory system. Twenty gills vascular outgrowths 

 of the body wall lie on each side of the thorax, sheltered 

 by the flaps of the shield. A current of water from behind 

 forwards is kept up by the activity of the baling portion, or 

 scaphognathite, of the second maxilla. Venous blood 

 enters the gills from the ventral sinus, and purified blood 

 leaves them by the six channels leading to the pericardium. 



Observed superficially, the gills look somewhat like 

 feathers with plump barbs, but their structure is much more 

 complex. The most important fact is that they present a 

 large surface to the purifying water, while both the stem 

 and the filaments which spring from it contain an outer 

 canal continuous with the venous sinus, and an inner canal 

 communicating with the channels which lead back to the 

 pericardium and heart. 



Three sets of gills are distinguishable. To the basal joints of the 

 six appendages, from the second maxillipede to the fourth large limb 

 inclusive, the podobranchs are attached. They come oft" with the 

 appendages when these are pulled carefully away, and each of them 

 bears, in addition to the feathery portion, a simple lamina or cpipodilc. 

 The membranes between the basal joints of the appendages and the 

 body, from the second maxillipede to the fourth large limb inclusive, 

 bear a second set, the arthrobranchs t which have no epipodites. In 



