VI COMPOSITION OF THE BLOOD I 7 I 



difference of opinion occurs with regard to the precise function of 

 the foot-pore which occurs in many MoUusca, 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 cor- 

 puscles but the liquor sanguinis — is colourless, or slightly tinged 

 with blue on exposure to the air. This is due to the presence 

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

 copper and iron, the former predominating. Haemoglohin, the 

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

 kester,^ 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 Flanorbis ; (3) in the muscles of the 

 pharynx and jaws of certain Gasteropoda, e.g. Limnaea, Paludina, 

 Littorina, Chiton, Aplysia. This distribution of haemoglobin is 

 explained by Lankester in reference to its chemical activity ; 

 whenever increased facilities for oxidisation are required, 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 Flanorhis is to be explained from its re- 

 spiring 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 oi 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 Aoxa 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, Trochus, Turlo, giving the parts the 

 appearance of raw meat. 



1 Proc. Roy. Soc. 1873, p. 70. 



- Griesbacli {Arch. mikr. Anat. xxxvii. p. 22) finds haemoglobin in several 

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

 glycimeris. 



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



