ORGANS OF THE SPAT 73 



when the shell itself is not firmly attached, but simply held firmly down to the substance 

 with which it is in contact, the young animal gets its food, or a portion of it, by means 

 of a sort of proboscis, or elongation of the mouth part, which is capable of being moved 

 about freely within the shell cavity. This proboscis stage lasts until the gills are formed 

 and become of sufficient size to supply food to the animals, when the proboscis, or rather 

 its flexible end, is transformed into the labial palps, which become closely connected 

 with the gill-leaves." 



Jackson ('90, pp. 301, 304-305, Plate XXIV, figs. 1-2) mentions palps in the 

 youngest spat stage and fortunately figures them. Long before I had prepared sections 

 of this stage I had decided that palps of such relative size could not be present, and that 

 what Jackson had was nothing but the unrecognized foot. His figures show it immedi- 

 ately behind the already shrunken velum and overlapped by the anterior gill-filaments 

 of both sides. The two transverse lines may have been due to its being crumpled or 

 else already shrunken, and the ventral furrow at the end may have suggested its being 

 double like two palps. It is not the same part as is figured as palps in his figs. 3 and 4. 



The mouth narrows down into an oesophageal tube of transversely 

 elliptical calibre, lined by ciliated columnar epithelial cells similar to and 

 continuous with those on the palps. The oesophagus is still relatively 

 long and curves over the anterior end of the stomach, passes between the 

 lateral origins of the liver, and enters the stomach from above. The 

 stomach is a comparatively large mass of irregular shape and large cavity, 

 occupying a good part of the space in the prodissoconch. In 1 mm. spat 

 (Plate V, fig. 33) there may be considered to be three prominent extensions; 

 one forwards below the oesophagus, another postero-dorsal behind the 

 entrance of the oesophagus, and a third, beginning as a compressed ven- 

 tral extension of the first, slants downwards and backwards towards the 

 great adductor muscle, becoming broad, deep, regular and thick-walled. 

 This, I am satisfied, is the portion that secretes the crystalline style, and 

 originates postero-dorsally in the left umbo of the larva, but becomes 

 moved to its present position during the rotation of the viscera, and pre- 

 sents the appearance, in the inside, of a regular, dense growth of cilia. 

 Just in front of the insertion of the oesophagus but on each side of it, i.e. 

 dorsolaterally, spring the stalks of the liver — one on the left and two on 

 the right. These branch into numerous follicles, lying on both sides of 

 and above the stomach, and projecting far forwards to the region of the 

 mouth. On the ventral aspect of the stomach, in the same region, springs 

 the intestine, on the right, in the crease between the compressed antero- 

 ventral extension to the left and the main central body of the stomach 

 above (Plate VII, fig. 6). From this the intestine passes backwards on the 

 right, then forwards behind the rounded postero-dorsal extension, forwards 

 and down the left side, where, near the anterior end of the stomach, it 

 turns backwards and then down, finally passing towards the median plane 

 to the anal opening over the adductor muscle. The stomach sometimes 

 contains diatoms and desmid-like clusters of one to four nucleated cells. 



During the growth into the larger spat the chief feature of the in- 

 testinal system is the downward prolongation of the part which gives rise 

 to the crystalline style below the adductor muscle. 



