STUDIES ON POLYCHAT LARVZ. 599 
what arbitrarily, and I have used it to denote a morpho- 
logical rather than a physiological stage of development. 
(5) The larva usually takes to its life on the sea bottom at 
the close of the nectocheta stage, but in the case of Polynoé 
at least the nectocheta stage is commonly regarded as ending 
after a period of partially arrested development, although the 
pelagic life is continued beyond this until several additional 
segments have been developed. 
In some Polychzets the metatrochophore is succeeded by an 
elongated larva in which swimming is effected by means of 
undulatory movements of the body; this form may be dis- 
tinguished from the Nectocheta under the name Nectosoma. 
A larva is said to be monotrochal when only the “ pre- 
oral” ciliated band is present, and telotrochal when it bears 
in addition the “ perianal” band. 
In larve of the Cheetopteridz the ciliated band is situated 
in a position which ultimately becomes separated from the 
head by several segments—such a larva is said to be 
mesotrochal. 
A segmented larva that bears several ciliated bands at 
intervals along its body is said to be polytrochal. 
A larva bearing no ciliated band is often referred to as an 
atrochal larva, though this term is not given in Prof. 
Hacker’s list. It should be pointed out, however, that this 
term has often been used to describe larve in which an 
extensive tract of short cilia encircles the body (as in the 
protrochophore and immediately succeeding stages of Hunicid 
larvee) ; such a larva is not atrochal in the restricted sense in 
which the term is used in the present paper. 
The “ preoral”’ ciliated band may be termed the proto- 
troch to distinguish it from any band situated between itself 
and the apical plate, such a band being called an akrotroch. 
Any band posterior to the prototroch is spoken of as a para- 
troch, and of these the “ perianal” band, situated upon the 
terminal segment (posterior to the formative region of the 
body), may be distinguished as the telotroch (Lankester; 
= “Endparatroch,” Hicker), from the remaining inter- 
