STUDIES ON POLYCHAT LARVA. 619 
form. Asa rule, too, the trochophore becomes a metatrocho- 
phore by the simultaneous appearance of several (“ primary ”’) 
body-segments whose appendages become fully developed 
before any additional (‘secondary ”) segments are added ; 
in a few species (e.g. Phyllodocid B of the Port Erin 
larvee) it is, however, impossible to distinguish a definite 
number of primary segments. ‘This character of the simul- 
taneous development up to an advanced stage of a number of 
anterior segments has become so engrained in the Nereidi- 
form stock that it very frequently happens that the parapodia 
of the first two or three segments develop more slowly than 
do those of the immediately succeeding ones instead of 
appearing before them. ‘This is most clearly shown amongst 
Port Erin larvee in the (undoubted) Syllids (see Gravely, 1909), 
but it is also very well marked in the case of Nephthys. 
In this way it comes about that until all the fundamental 
parts of the appendages have appeared the larva remains a 
rather short, stout creature capable of little of the wriggling 
movement that more fully developed worms exhibit. The 
appendages of Fewkes’ Nephthys larva (Fewkes, 1885, pp. 
180-184, Pl. IV, figs. 1-12b) apparently acquire their full 
complement of parts in the 10-segment stage; the segments 
of larve belonging to this genus are less compressed than is 
frequently the case in the early stages of Nereidiform worms; 
and this larva has a more elongated form than any other that 
I know of at the period of the appearance of the last-developed 
parts of the parapodia—which, moreover, does not take place 
until after the development of a number of secondary seg- 
ments.! 
The only case at present known in which all the segments 
of a Nereidiform larva are formed in succession and of con- 
siderable length would appear to be that of Ophryotrocha 
(see Braem, 1893, pp. 219-220, Pl. XI, figs. 33-86, and 
1 Fewkes himself does not distinguish between primary and secondary 
segments, but in the Port Erin species of Nephthys at least it is 
certain that the former begin to appear before the gill and cirrus, 
which have not been seen on any larva yet examined. 
