Respiration in Invertebrate Animals. 1 87 



to him, the same straight vessel {p, a), and funiished too wath 

 valves opening in one direction, sends the blood in two diame- 

 trically opposed currents ! forwards towards the head, and 

 directly backwards towards the tail at the same moment*. He 

 thus wantonly violates the unity of principle which presides over 

 the distribution of the blood in the whole annulose and arti- 

 culated series. 



Directly contrary to the views of Mr. Newport, the blood in 

 truth, in the caudal arterij (fig. ^,p) of the Arachnid, moves 

 forvmrds, not backwards. It follows therefore, that in the ventral 

 trunk (/) of the tail of the Scorpion the direction of the current 

 is backwards, not forwards as stated by Mr. Newport. The 

 pneiuno-cardiac channels {k, k, k) enter the auricles of the heart ; 

 the systemic arteries ai-ise (Newport) from almost the same point 

 in the walls of the chambers. The author has already a,rgued 

 in favour of the venous character of these vessels in jNlp-iapods 

 and Insects : if in the latter classes they be venous, they cannot 

 be arterial in the Scorpion; they are the same vessels— they 

 present the same relations — they are connected with the viscera 

 in the same manner. They anastomose with the branches which 

 proceed upwards from the supraspinal visceral arteries. Hence- 

 forth they will be called systemic veins. 



At its cephalic extremity in the Scorpion, the dorsal vessel 

 divides into three groups of secondary trunks (fig. 3, b, c, d), — 

 those first which supply the brain, head and tentacles, — those 

 secondly which proceed to the claws and legs, — and lastly those 

 which form the great ventral longitudinal trunks of the body. 

 In the Scorpion they consist of two orders, those first which 

 Mr. Newport in this instance has called the visceral arteries {b), 

 and secondly the supraspinal artery (c). If the visceral arteries 

 exist as separate trunks in Arachnids, they must be present 



* This is his language : — " Having traced the distribution of the arterial 

 vessels from the anterior extremity of the heart, it remains now to follow 

 those of the posterior, which afford some curious peculiarities. The last 

 two chambers of the heart, which are situated in the seventh segment of 

 the abdomen, are greatly reduced in size, and constitute the origin of the 

 caudal artery (fig. 3, p, of the author's plate), and seem to be the means 

 by which part of the current of the blood is directed backivards to the tail." 

 See page 292 of his paper on the MjTiapods and Arachnids, in the Phil. 

 Trans. 1843. There are eight valves to the heart. The anterior six act 

 forwards. The two posterior act directly backwards (Newport) !— Is this 

 probable, physiologically or mechanically ? Does it not involve a hydraulic 

 absurdity ? Can the same linear tube, whose contractions begin behind and 

 travel undulatorily/ortcards, drive the contained fluid simultaneously back- 

 wards and forwards? (!) Why should this reversal of the blood-current 

 take place in the same homologous vessel in the Arachnid, and not in the 

 Insect and the Myriapod? The mere addition of a tail to the Scorpion 

 does not necessitate such a mechanical paradox. 



