ANATOMY OF THE PEARL OYSTER. 53 



With the exception of the heart, and the somewhat indistinct appearance of 

 striation in the larger portion of the adductor, the muscle fibres throughout the body- 

 are non-striped. 



THE ALIMENTARY CANAL. 



As the oesophagus and the stomach, together with two-thirds of the intestine, lie 

 within the viscero-pedal mass (see Plate VI., fig. 1), the surrounding tissues, first 

 those of the superficially placed gonad and then of the more deeply lying digestive 

 gland have to be carefully picked away before the relative positions and the course of 

 the parts can be traced. Hardened material is easier to manipulate than fresh ; and 

 the best method is to kill the animal by immersion in a 5-per cent, aqueous solution 

 of formol, and to keep it therein till the day prior to dissection, when it should be 

 removed from the shell and soaked in several changes of fresh water to remove most 

 of the formol. 



Two projecting horizontal lips conceal the aperture of the mouth (0., Plate VI., 

 fig. 1). Each is smooth on both outer and inner surfaces, and is produced laterally 

 at each extremity into a labial palp, the upper lip passing right and left into the right 

 and left dorsal palps, and the lower into the corresponding ventral palps (Pa., in 

 various sections on Plate V.). The palps are smooth on the surfaces turned away from 

 the mouth, but are closely grooved on the opposed faces which bound the entrance to 

 the mouth. The mouth thus guarded lies at the base of the deep cleft formed by the 

 approximation of these lips. It is a large, slitdike depression placed transversely 

 between the anterior levator muscles of the foot. Each of the two corners or angles 

 of the mouth is produced laterally to merge imperceptibly into the palpar gutter that 

 marks the line of junction of the dorsal and ventral palps of that side. In this way 

 a long shallow ciliated gutter leads up to each angle of the mouth from between the 

 palps. 



The oral cavity rapidly contracts inwards to the narrower width of the oesophagus 

 (Oc, Plate VI., fig. 1), which is a short, straight, ciliated tube, dorso-ventrally com- 

 pressed, continued posteriorly along the median line and in the same plane as the 

 mouth. Its hinder end opens into the anterior end of the stomach, slightly below 

 the level of the roof. 



The form of the stomach (St., Plate VI., fig. 1) is ovoid, the long axis lying 

 horizontally, with the narrow end directed anteriorly and slightly upwards. It is 

 unsymmetrically placed, encroaching greatly upon the left portion of the visceral 

 mass so much so that three-fourths of its capacious chamber lies to the left of the 

 median plane. Except at the extreme left, and for a small space on the roof, it is 

 enveloped by the digestive gland. 



Folds and depressions diversify the walls and floor of the stomach and break them 

 up into definite areas. The most conspicuous is a slightly projecting vertical fold 

 which arises from the posterior wall and from the hinder part of the floor, marking 



