462 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
viduals. Each of the two spiral stigmata which wind around the wall 
may in a long infundibulum make ten or more complete turns between 
the base and the apex, but every intermediate stage between such a 
condition and a very low conical infundibulum with only a few turns 
can be seen on the same individual; and here and there rudimentary 
or incipient spirals in which each stigma makes only a part of a turn, 
are scattered among the fully developed ones. Apparently it is only 
in localities exceptionally well adapted to the growth of this animal 
that great complexity of the branchial sac is attained. In other 
environments the growth of the body is more or less stunted and the 
number of infundibula developed is much fewer. In such specimens 
and in young individuals it can be seen that there is a definite system 
in the arrangement of the infundibula and in the way their spirals are 
connected together. The infundibula (and the spirals formed by 
their stigmata) may in such individuals be seen to be of two kinds, 
here called primary and secondary. The primary' spirals appear first 
in the young animals, have a definite situation and direction of 
twist, and for a long time are conspicuous for their greater number 
of turns, and it is only in large and old individuals that the secondary 
spirals and infundibula afterward developed attain numbers and 
dimensions sufficient entirely to obscure this regularity. 
In describing the arrangement, the spirals alone will be alluded to, 
it being understood that as a spiral becomes well developed, that part 
of the wall of the sac which it occupies becomes raised into an in- 
fundibulum. Leaving out of account for the present the spirals in 
the spaces dorsal to the first internal longitudinal vessel and those 
bridged over by the seventh (most ventral) internal longitudinal 
vessel, the apex of each primary spiral is nearly or directly under 
(bridged over by) an internal longitudinal vessel, and is situated at a 
point midway along one of the segments into which that vessel is 
divided by the transverse vessels. Thus, although the vessels di\'ide 
the surface of the sac into conspicuous fields, the systems of connected 
spirals do not correspond to these fields, since the primary spirals lie 
partly in one field and partly in the field next dorsal to it. In young 
individuals, the primary spirals may be the only ones. But as the 
individual grows, other spirals {secondary spirals) begin to develop. 
Usually the two first secondary spirals form ventral to the primary 
one with which they are connected. As the animal grows other 
secondary spirals (all forming part of the same chain or series) develop 
