be 
MUSEUM OF COMPARATIVE ZOOLOGY. 169 
opening into a large anterior vessel. The pulsatile sac resembles a heart ; 
the vessel opening from it, an artery which may distribute blood to the head 
and cephalic tentacles. Of the true homology of these organs there is, how- 
ever, some doubt. 
The body of the youngest Prionospio (Fig. 1) is composed of nine anterior 
segments, bearing as many pairs of long provisional sets and four smaller 
terminal segments without spines. Consequently, it will be seen that in the 
youngest larva two segmented regions can be distinguished in the body ; 
the anterior (ar) forming its great mass and bearing provisional sete, and 
the posterior (yr) relatively almost inconspicuous in size and without spines. 
The terminal segments of the latter are colored by bright red pigmentation. 
The diameter of the intestinal tract narrows uniformly from the head to the 
anal extremity, with little variation in different regions. 
Marked changes of most important character have taken place in the head 
and body of the next oldest larva (Figs. 2, 3,4, 5). The arrangement of the 
tentacles, setee, and eye-spots on the head is about the same as in the former 
larva, and the dorsal walls have extended forward above the mouth into a lip 
which had a rounded border, forming a structure which persists into the adult, 
and will be called, in subsequent larvee of this worm, the preoral lobe. This 
nomenclature, however, does not imply that it is homologous with the struc- 
ture which has the same name in certain other Annelid larvae. The body of 
this larva has dropped, either normally or abnormally, most of its embryonic 
sete, and three regions, an anterior, a middle, and a posterior, have differentiated 
themselves in it. Almost the whole of the body is still taken up as formerly 
by the anterior region. The middle region (mr) is smaller than the anterior, 
has its walls more thickly pigmented, and retains the embryonic sete even 
when the larva is kept in confinement for some time. This region is formed 
from the originally undivided posterior part of the former larva. The pos- 
terior body region is the smallest of the three, and is the same as the non- 
spinous part of the body of the youngest larva. 
The anterior region of the body in the present larva is composed of nine 
segments, the lines of separation between each pair of which, however, are not 
well marked. The lateral spines of this part are short and small. The body 
walls are very transparent. On the sides of the body near the fourth pair of 
spines there is a cluster of reddish pigment spots (ms), which persist even into 
the oldest larvee which have been taken. (Fig. 13). 
The middle body region, which is developed from the original posterior por- 
tion, is formed of four segments, the constrictions between which are deep 
and well marked. The segments are sometimes swollen to a diameter 
greater than that of the transparent anterior portion of the body. The walls 
are thicker than those of the transparent part described above, and are more 
densely pigmented with yellow and brown. That portion of the digestive 
tract which lies in the middle body division is here considered the stomach. 
The posterior division of the body has a smaller diameter than either of the 
others, and is without appendages. It is, however, segmented, and later in its 
