CIRCULATION. EXCRETORY ORGANS. 353 



segment, through which the blood enters from the surrounding 

 pericardium. A terminal ostium is situated at the posterior 

 end of the heart and the blood is propelled forwards by rhythmic 

 contractions. The " aortic " artery leading forwards from the 

 anterior end opens into the system of lacunae which pervades 

 the body, especially the superficial regions beneath the integu- 

 ment. In the limbs the lacunae are so arranged that the blood 

 flows down one side and up the other, supplying the epipodial 

 branchiae in its course. An incomplete transverse septum 

 dorsal to the alimentary canal separates the pericardial sinus 

 from the lacunae ventral to it. 



The variants on this arrangement met with in other groups 

 of Crustacea consist mainly in the shortening of the heart and 

 pericardium in various degrees (until we reach the capsular 

 form of heart found in the Euphausiidae and Decapods on the 

 one hand, and the Cladocera on the other) and in the develop- 

 ment of the system of arteries between the heart and haemocoele. 

 This system is most complete in the Decapods where the trunks 

 become subdivided into arterial capillaries. A remarkable vessel 

 found in this group, and in the Schizopods and Stomatopods is 

 the sternal artery which passes ventrally from the heart on one 

 side of the intestine and between the parallel strands of the 

 nerve cord to communicate with the subneural artery from 

 which the limbs are supplied. 



In Decapods the gills are supplied by a system of lacunae 

 independent of the vessels to the limbs, an afferent set leading 

 from the large cephalo-thoracic sinus to the gills, and an efferent 

 conducting the blood to the pericardium, where it mixes with 

 the venous blood returned from other parts. In this group 

 the blood is propelled backwards as well as forwards, a superior 

 abdominal artery extending backwards from the heart above 

 the intestine. As the heart is abbreviated in the several groups 

 the number of ostia diminishes. 



The excretory organs usually consist of a more or less coiled 

 tube ending internally in a sack and opening at the base of an 

 appendage. They are probably homologous with the segmental 

 tubes of Annelida and other coelomate animals. In the Mala- 

 costraca the antennal gland (the green gland of Astacus) opening 

 at the base of the second antenna is usually the excretory 

 organ of adult life, though the maxillary gland is found in the 



z in A A 



