VI COMPOSITION OF THE BLOOD I71 



difference of opinion occurs with regard to the precise function 

 of the foot-pore which occurs in many Mollusca, some holding 

 that it serves as a means for the introduction of water into the 

 blood-vascular system, while others regard it as a form of secretion 

 gland, the original purpose of which has perhaps become lost. 



Blood. — As a rule, the blood of the Mollusca — i.e. not the 

 corpuscles but the liquor sanguinis — is colourless, or slightly 

 tinged with blue on exposure to the air. This is due to the pres- 

 ence of a pigment termed haemocyanin., in which are found traces 

 of copper and iron, the former predominating. Haemoglobin^ the 

 colouring matter of the blood in Vertebrates, is, according to 

 Lankester,^ of very restricted occurrence. It is found — (1) in 

 special corpuscles in the blood of Solen legumen (and Area Noae) ; 

 (2) in the general blood system of Piano rhis ; (3) in the muscles 

 of the pharynx and jaws of certain Gasteropoda, e.g. Limnaea^ 

 Paludina., Littorina., Chiton., Aplysia. This distribution of hae- 

 moglobin is explained by Lankester in reference to its chemical 

 activity; whenever increased facilities for oxidisation are re- 

 quired, then it may be present to do the work. The Mollusca, 

 being as a rule otiose, do not possess it generally diffused in 

 the blood, as do the Vertebrata. The actively burrowing Solen 

 possesses it, and perhaps its presence in Planorhis is to be 

 explained from its respiring the air of stagnant marshes. Its 

 occurrence in the pharyngeal muscles and jaws of other genera 

 may be due to the constant state of activity in which these 

 organs are kept.^ 



According to Tenison- Woods ^ a species of Area (trapezia 

 Desh.) and two species of Solen., all Australian, have red blood. 

 It is suggested that in these cases the habits of the animal (the 

 Solen burrowing deeply in sand, the Area in mud) require some 

 highly oxidising element, surrounded as the creature is by ooze. 

 In Area pexata (N. America) the blood is red, the animal being 

 familiarly known as the 'bloody clam.' Burrowing species, 

 however, are not all distinguished by this peculiarity. Tenison- 

 Woods finds red fluids in the buccal mass of many Gasteropoda, 

 e.g. in species of Patella^ Acmaea, Littorina, Troehus, Turbo, 

 giving the parts the appearance of raw meat. 



1 Proc. Boy. Sac. 1873, p. 70. 



2 Griesbach {Arch. mikr. Anat. xxxvii. p. 22) finds haemoglobin in several 

 bivalves, e.g. Poromya granulata, Tellinata planata, Area Noae, and Pectunculus 

 glycimeris. 



^ Trans. Boy. Soc. N. S. Wales, xxii. p. 106. 



