352 CRUSTACEA. 



posterior end of the mid-gut, but the excretory function attri- 

 buted to it is doubtful. In any case it is not comparable 

 with the malpighian tubes of Insects, which are diverticula of the 

 hind-gut. The hind-gut in Crustacea is without appendages. 



The peristaltic contractions of the gut subserve the circu- 

 lation of the blood, and it appears to be the chief agent for this 

 purpose in the small Entomostraca which are without a heart. 



The chitinous linings of the fore- and hind-guts are shed at 

 each ecdysis. The remarkable paired calcareous concretions, 

 known as crabs' eyes and found in the walls of the stomach of 

 Decapods are also shed at this time and, being ground up by 

 the stomach, apparently furnish the lime salts for the hardening 

 of the new external cuticle. 



In the strangely distorted and blood-sucking females of some 

 parasitic Isopods (Epicarida) the alimentary canal ends blindly, 

 as it does in some aberrant Cirripedes. In the Rhizocephala 

 and adult Monstrillidae the canal is absent altogether. 



The blood of Astacus and of other Decapod Crustacea and of 

 Squilla 'is a clear fluid containing haemocyanin, a respiratory 

 substance in which copper is present in combination with a 

 proteid. It is colourless when deoxidized, but bright blue 

 when oxidized (oxyhaemocyanin). A red substance may also 

 be present " tetronerythrin " but it is doubtful if the latter 

 possesses a respiratory function. In the Branchiopods Daphnia 

 and Chirocephalus, however, Lankester found that the respiratory 

 substance of the blood plasma is haemoglobin, and this is the 

 case also in Apus, Cypris, and in the parasitic Copepod Lernan- 

 thropus.* Floating in the plasma are colourless amoeboid 

 corpuscles. 



The Crustacean circulatory system consists of a heart (absent 

 however in the Cirripedes and many Ostracods and Copepods) 

 lying dorsal to the alimentary canal, a system of arterial vessels 

 more or less extensive, and of the lacunar haemocoele, of which 

 the pericardium is part. 



The Phyllopod Branchipus (Fig. 247) possesses a condition 

 of the circulatory system, which may well be regarded as primi- 

 tive. The heart is a long vessel traversing the thorax and 

 abdomen, and provided with a pair of valvular ostia in each 



* Cf. Halliburton, Blood of Decapoda, Journ. of Physiology, vol. vi. 

 1885. 



