and sometimes only two are observable. An appearance as of a dorsal blood-vessel with lateral branches is, in prepared specimens, very constant at this stage; but it seems unlikely that such a vessel would yet be established (fig. 2). I did not observe this appearance in living specimens. Two eyes, reddish in colour, are noticeable. There is no trace of tentacles or of anal appendages. A lateral view of a prepared specimen at this stage shows an opening behind each chætigerous sac: these openings are probably nephridiopores. When four segments follow the peristomium a pair of palps is developed, and the rudiments of a pair of tentacles and of a pair of anal cerci appear (fig. 3). There is no indication of an unpaired tentacle. The notopodium and neuropodium of the appendages of the first pair, those of the peristomium, lose their chætæ and develop as jointed organs with a few rigid hairs. These are to be the peristomial tentacles. In connection with the appendages of each pair is now a pair of flagella. In the case of the peristomium these appear to represent the notopodial cirrus; in the case of the other appendages, the neuropodial cirrus. The egg membrance has by this time disappeared, and the gelatinous matter in the neighbourhood of the larva deliquesces. In this liquid area, which extends constantly, the larva swims by means of its flagella. I am not certain when the flagella disappear; but I could not detect them in any larva that had developed eight segments. Two pairs of small eyes have appeared on the peristomium. In this stage also, that in which four complete segments follow the peristomium, stomodæum and proctodæum appear to develop. A little later the jaws are observable, and still later they become very noticeable and may be seen to be carried forward with the pharynx, snapping vigorously. The eyes on the prostomium, the first pair that appeared, usually disappear by the time six segments are developed. Just as the parts of the parapodia of the peristome lose their chætæ, so does the notopodium of the next segment lose its chætæ and develop as a jointed, tactile organ. I saw no change in the neuropodium of this segment (figs. 5 and 6). I observed the development of several specimens up to thirteen segments; but beyond that time I was unable to keep them alive. Explanation of Plate XXIII. Fig. 1. Egg-mass; × 1 ½. Fig. 2. Dorsal view of larva with three pairs of chætigerous appendages. Cœlomic pouches developing.