192 PHYSIOLOGY OF THE INVERTEBRATA. 



with the serous membrane, that invests the heart of the 

 Vertebrate animals. It is, in truth, a large venous sinus, 

 surrounding that long segmented vessel in the dorsal region 

 of the body that is generally called the heart. From this 

 sinus, blood passes into the heart by certain lateral openings 

 provided with valves opening inwards. Yet another unfor- 

 tunate name has been used in this connection. Certain parts 

 of the venous system in the Insecta and Mytiapoda have been 

 designated portal. They represent, however, in no manner 

 the portal system peculiar to the Vertehrata." 



In the Arthropoda, there are no pseudo-hsemal vessels ; and 

 " the blood-vascular system varies from a mere perivisceral 

 cavity without any heart (Ostracodct , Cirripedia) up to a 

 complete, usually many-chambered heart with well-developed 

 arterial vessels. The' venous channels, however, always have 

 the nature of, more or less, definite lacunfe. The blood cor- 

 puscles are colourless, nucleated cells."* 



In all those Arthropods where a heart is present, the blood 

 returns to that organ by the lacunar spaces situated between 

 the organs. These condaits, without special walls, debouch 

 into a so-called pericardiacal reservoir, and the blood pene- 

 trates afterwards into the heart by cardiacal clefts. In the 

 Brachyura and Macroura (Fig. 39) the blood, before returning 

 to the heart, is oxidised in passing through the branchiae. 



In the Myriapoda, the heart has many chambers, and it is 

 nearly as long as the body. The blood enters this organ by 

 a pair of clefts, and leaves it partly by the communication 

 with the adjacent chamber, and partly by the lateral arteries. 

 "A median aortic trunk continues the heart forwards, and the 

 lateral trunks encircle the oesophagus and unite into an artery 

 which lies upon the ganglionic chain. The arterial system 

 in the CJiilopoda is, in fact, as complete as that of the 

 Scorpions."t 



In the Insecta, circulation is chiefly effected by means of 



* Huxley's Anatomy of the Invertehrata, p. 252. 



t See Newport in the FhilosopMcaVTransactions of the Boyal Society, 1863. 



