STUDIES. ON POLYCHAET LARVA. 621 
due largely to the correlation of larval with adult structure, 
and there are no other characters at present known by which 
these larvee can be recognised as forming a distinct group, 
we have as yet no embryological evidence really independent 
of that obtained from the adult as to the manner in which 
these worms have been derived from a more primitive vermi- 
form stock. 
However, in the investigation of larve belonging to 
separate families of the Nereidiformia, structures of greater 
interest from a systematic point of view have already been 
found. ‘The families whose pelagic larve are most completely 
known are the Polynoide and the Phyllodocide. In both of 
these all known Trochophores show well-marked larval 
structures that appear to be exclusively characteristic of the 
family to which they belong. In the Polynoide these are 
the overhanging upper lip and the associated oral ciliary 
apparatus, the transverse akrotroch (a simple structure, how- 
ever, that is likely to be found in other families when their 
Trochophores are better known), and perhaps also the 
circlet of apical cilia (see Gravely, 1909), though Hicker 
figures these cilia in a tuft (1894, Pl. XIV, fig. 1; 1896, Pl. 
III, fig. 2) or series of tufts (1896, Pl. II], fig. 1). Trocho- 
phores of the Phyllodocidz on the other hand are charac- 
terised chiefly by their great contractility and by the ventral 
hook?’ of e1liat! 
The Eunicidz appear to be characterised by the omission 
of the true trochophore stage. As noted above this has 
been shown by Hicker to be due to the early development of 
these larve taking place under the protection of a gelatinous 
capsule. 
The strictly larval characteristics of other families of the 
Nereidiformia have yet to be determined. 
1 Hiicker has defined the characters by which Phyllodocid trocho- 
phores may be recognised (1896, pp. 84, 85); the cilia by the eyes are 
not, however, present in all species (see Gravely, 1909), and must certainly 
be removed from the list of characters distinguishing them from those 
of the Polynoide (Hacker, loc. cit., p. 85; see Gravely, loc. cit.). 
