ORGANS OF THE LARVA 49 



Gills originate in this period of development. They begin as a 

 ridge along each side of the body, underneath the mantle and above the 

 base of attachment of the foot (Plate V, figs. 30-32; Plate VI, figs. 4, 5). 

 Each ridge soon becomes segmented by external transverse creases 

 into a series of short papilla-like processes, diminishing in size from before 

 backwards — the last ones being mere knobs. In the oldest free-swimming 

 larva there are about eight filaments in a series, the left being best de- 

 veloped; their lower ends free, but their upper ends joined in the axis 

 that extends backwards and is united with its mate of the opposite side 

 porteriorly behind the foot. They are at first solid but later become 

 hollowed from above. These two series continue to develop into the right 

 and left inner series of filaments of the adult oyster. 



Adductor Muscles extend transversely between the valves of the 

 shell (Plate V, figs. 30-32; Plate VI, figs. 1, 6), an anterior, at first 

 larger, one in front of and above the velum, and a posterior one below the 

 hinder parts of the umbos, which becomes the single great permanent 

 adductor of the adult oyster. The anterior adductor was known to 

 Huxley (1883), who argued that it could not be the permanent 

 adductor. The posterior adductor was discovered by Jackson (1888). 

 Retractor muscles converge from the umbos to the velum and to the foot 

 and there are intrinsic muscle-fibres in the velum, the foot and the mantle. 



The Intestinal Canal. — (Plate V, fig. 31) increases until it is much 

 longer than the body and in consequence has to become folded, the greater 

 part lying left of the median sagittal plane, but mouth, oesophagus and 

 anus are median. The mouth is a funnel-shaped opening lying imme- 

 diately below and behind the velum, to which its walls are attached and 

 with which it is protruded and withdrawn; so that it can only be functional 

 when the shell is gaping and best when the velum is to some extent ex- 

 panded, the activity of its cilia perhaps contributing to the process of 

 feeding. In sections, cilia of the inside of the folded velum point in- 

 wards to the mouth. The oesophagus lies between velum and foot in the 

 median sagittal plane as well as in or very near to the median transverse 

 plane of the body. Here it passes towards the dorsal region, between the 

 first gill filaments of opposite sides and opens into the stomach which is 

 provided with large, brown, lateral liver-sacs. The intestine passes back- 

 wards towards the right and then forwards towards the left, v/hen it again 

 turns backwards and upwards in the left umbo and finally, as rectum, 

 downwards in the median plane, over the heart and posterior adductor 

 muscle to the anus. . 



Pigment Spots (eye-specks) appear in larvse of about 40 units length 

 — one on each side — as conspicuous black spots, Viewed from the side 

 (Plate V, figs. 30, 32) they appear to be about the centre of the animal, 

 anterior to the gills. In sections (Plate VI, figs. 3, 5) they are found to 



