189 
in which all the gills are present, and it would therefore 
appear that most specimens take to a littoral habit rather 
earlier than this one. In point of length it is not, how- 
ever, a large specimen, being only 3°9 mm. long. In life 
it would certainly be longer; this measurement is taken 
from the mounted specimen, which was unfortunately 
preserved in a somewhat contracted condition. The 
longest post-larval stage I have seen measures 6°5 mm. 
from anterior to posterior ends, and this one has no gills. 
Evidently, therefore, the time of appearance of these 
organs varies in different specimens. Perhaps the two 
specimens under consideration belong to the two different 
varieties of A. marina, but there are no means of establish- 
ing this point in the larger specimens; the smaller almost 
certainly belongs to the littoral variety. 
Nothing is known concerning the duration ot 
the pelagic lite of these post-larval stages, but it is 
apparently at any rate some days, judging from the 
varying sizes of the specimens captured within a few days 
of each other. For example, in a series of eight speci- 
mens taken near Plymouth during the last days of March 
and early days of April, 1901, there are specimens varying 
from 3°5 to 65 mm. in length. Again, the varying sizes 
of these specimens, and the length of the period during 
which they may be taken in the tow-nets, show that the 
spawning period of A. marina may occasionally extend 
over two or three months, e.g., post-larval stages have 
been taken as early as March 8th, and as late as May 29th. 
The latter date is, however, exceptionally late; most 
specimens are taken either in March or in the early days 
of April. The smallest worms I have found in the sand 
were taken near the end of June, and are 17 mm. long. 
They possess thirteen pairs of well-branched gills, the 
prostomium has assumed the adult form and relations, 
