536 THE ANIMAL KINGDOM. 



and is a simple oval sac with two venous inlets and a short anterior aorta 

 in Cladocera, some Ostracoda and Copepoda. It is long, thoracic in position 

 in Amphipoda, abdominal in Isopoda ; long and extending from the thorax 

 into the abdomen in Stomatopoda ; short in Podophthalmata, where it lies 

 in the thorax. It lies in a pericardial sinus in Arthrostraca and Thorac- 

 ostraca, with which it communicates by four paired ostia in Isopoda, three, 

 but sometimes fewer in Amphipoda : by a large anterior pair and twelve 

 smaller posterior pairs in Stomatopoda : and in the Podopthalmata by two 

 pairs in Mysis and three pairs in Decapoda. It usually gives off in the 

 higher Crustacea one or more anterior vessels to the eyes and antennae, 

 lateral to the glands of the mesenteron and posterior into the abdomen : 

 and in the Stomatopoda the posterior part gives off thirteen lateral pairs of 

 vessels. The system of vessels is especially well developed in Isopoda, 

 where the heart gives off lateral vessels to the feet. The anterior aorta in 

 Arthrostraca forms a peri-oesophageal vascular ring, from which in Isopoda 

 a sub-neural vessel is continued backwards giving off lateral branches. 

 The venous blood-spaces are limited by connective tissue in Arthrostraca 

 and Decapoda. In the three parasitic Copepodan genera, Ler nan tkr opus, 

 Clavella, and Cyonus ( = Congericola), there is a special system of tubes con- 

 taining a plasma with haemoglobin in solution. Respiration may be 

 partly cutaneous as in Copepoda and Cirripedia. The bivalve shell of 

 Cladocera, Ostracoda, and Nebalia is penetrated by the blood currents. 

 But in most Crustacea organs of respiration exist in the shape of ap- 

 pendages to the thoracic limbs (Branchiopoda, Cladocera, Leptostraca, Am- 

 phipoda, Decapoda) ; to the abdominal limbs {Stomatopoda) ; and sometimes 

 to the wall of the thorax itself (Decapoda}, though in this case it is probable 

 that their position is secondarily acquired. The endopodite of the ab- 

 dominal limbs, with the exception of the first pair which form an oper- 

 culum, is respiratory, thin, and transversely folded in Isopoda : and in the 

 genus Tylos belonging to this order a special process of the four anterior 

 pairs of abdominal feet bears cross folds with linear slits, which lead to 

 branched air sacs. The inner wall of the branchiostegite in Decapoda is 

 very thin, and probably respiratory, and in some land crabs there are 

 vascular growths of the branchial chamber. The exopodite of the thoracic 

 limbs in many instances, e.g. Phyllopoda, or of the second maxilla (De- 

 capoda), forms a broad plate which keeps a current of water in constant 

 motion. A rhythmical opening and shutting of the anus, admitting and 

 expelling water, and thus constituting an anal respiration, takes place in 

 Phyllopoda, Branchiopoda, and Cladocera, in Copepoda, Astacus, some Nauplii 

 and Zoaeae, and is probably very general. 



Excretory organs exist as the shell gland of Phyllopoda and Copepoda 

 which opens close to the second maxilla : the antennary gland aborted in 

 the two orders named during development, but persistent in Malacostraca, 



