N.o., Nephridial opening.
With 23 to 25 pairs of claw-bearing legs, four spinous pads on the legs, and nephridial openings of legs 4 and 5 in the middle of the proximal pad or on its proximal side; feet with two primary papillae, one anterior and one posterior; outer jaw with two, inner jaw with two or three minor teeth at the base of the main tooth, separated by a diastema from the row of small teeth; crural glands present in the male only, in the two pairs of legs preceding the generative opening; coxal glands present.
With 14, 15 or 16 pairs of claw-bearing ambulatory legs, with three spinous pads on the legs, and nephridial opening of the 4th and 5th legs on the proximal pad; feet with one anterior, one posterior and one dorsal primary papilla; inner jaw without diastema, outer with or without a minor tooth.
A and feet with two primary papillae on the anterior side and one on the posterior side; outer jaw with one minor tooth at the base of the main tooth, inner jaw with no interval between the large tooth and the series of small ones; last fully developed leg of the male with enlarged crural gland opening on a large papilla placed on its ventral surface; coxal organs absent; the nephridial openings of the 4th and 5th pairs of legs are placed in the proximal spinous pad.
In the same figure the free part of the inner lamella of the inner gill-plate resting on the foot is marked z, whilst the attached parjt - the most anterior - has been snipped with scissors so as to show the genital and nephridial apertures x and y.
Here again the nephridial funnel seems to have been converted into or certainly replaced by a secondarily developed funnel.
Ne.f., Larger nephridial funnel.
The genital duct opens by a pore into the urino-genital groove of the oyster (the same arrangement being repeated on each side of the body) close to but distinct from the aperture of the nephridial canal.
At the least there is no doubt that the genital funnels are developed precisely where the nephridial funnels formerly existed.
The internal vesicle is already indicated, and is shown in the diagram by the thinner black line: I, gut; 2, somite; 2', nephridial part of coelom; 3, haemocoele; 3', part of haemocoele which will form the heart - the part of the haemocoele on each side of this will form the pericardium; 4, nerve-cord; 4, slime glands.