Kirk. — Development of a Polychaete. 287 



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 chsetigerous 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 chsetse and develop as jointed organs with a few rigid 

 hairs. These are to be the peristomial tentacles. In connec- 

 tion 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^membrane has by this time dis- 

 appeared, and the gelatinous matter in the neighbourhood of 

 the larva deliquesces. In this liquid area, which extends con- 

 stantly, 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, stomodseum and proctodeum 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 pro- 

 stomium, 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 chsetse, so does the notopodium of the next segment lose 

 its cheetee 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 ; x 1|. 



Fig. 2. Dorsal view of larva with three pairs of chsetigerous appendages. 

 Ccelomic pouches developing. 



