1922] MADRAS PEARL FISHERIES 121 



removed, we have the appearance I have drawn in figure 4. 

 Occupying the hinder portion of the centre, is the great crescentic 

 fleshy mass of the adductor muscle, which passes athwart the body 

 from valve to valve and by its contraction closes the shell. Beneath 

 and to the front is a conspicuous striated sickle-shaped organ, the 

 gills or branchiae. Above the adductor and the gills is the swollen 

 yellowish visceral mass — the body proper. In it are contained 

 the greater part of the alimentary canal and of the nervous system 

 together with the main glands, those connected with digestion (the 

 so-called liver) and with reproduction. Behind this mass is a 

 triangular cavity — the pericardium--bounded below and behind 

 by the adductor muscle ; in it is lodged the heart, made up of two 

 dark pouch-like auricles below, and one yellowish strong muscular 

 ventricle above ; its upper wall enwraps for support a section of 

 the intestine, which thus has the appearance of passing through 

 the heart. From the anterior side of the visceral mass a tongue- 

 shaped organ, the foot, protrudes, passing beyond the shell, when 

 extended, through a small gap, the byssal sinus, between the two 

 valves just under the anterior " ear " of the shell. The foot is of 

 great importance to the pearl oyster; by its aid it can drag itself 

 slowly along when it desires to shift position and also can attach 

 itself securely to some stable foothold. The foot is highly caver- 

 nous, full of spaces that can be injected and inflated with blood 

 at will. Along the lower surface is a groove ending behind in a 

 deep pit. If the oyster has detached itself from its previous 

 foothold and desires to crawl, the foot is injected with blood and 

 the tip pushed forward to its full extent, while the sides of the 

 groove at the fore end flatten and produce an adhesive surface. 

 On this purchase, the animal drags itself forward and then while 

 retaining a hold at the hinder end of the foot releases the tip and 

 pushes forward another quarter of an inch. Progress is slow and 

 any continuous journey seldom exceeds a few inches. When the 

 animal desires to refix itself, the foot is again employed for the 

 purpose. In this case, the sides of the groove come together and 

 form a temporary tube, except at the fore end where a small space 

 remains open upon the surface to which attachment is to be made. 

 The cells lining the groove now pour forth a gluey secretion which 

 sets firm and elastic and takes the form of the tube within which 

 it was produced. At the end of about five minutes, the byssal 

 groove opens and reveals a stout golden green fibre ending at 

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