1891.] A Commensal Annelid. 29 
as a heart, since it contains peculiar branching, nucleated proto- 
plasmic processes suggesting the cells of an embryonic vertebrate 
heart, and to be interpreted as part of a mesenchyme, formed, like 
the blood, outside the ccelom. The ventral side of each branchia, 
that next the animal’s back, bears along it a band of large ciliated 
cells, each with a large tuft of fine cilia. A similar row of such 
cells is found extending transversely across the back of each 
somite, and corresponding nearly to the lines of attachment of the 
internal septa. The branchia begin upon the sixth somite 
(Fig. 7), and are present, though very small, at the posterior end 
of the body (Fig. 8). 
The setz in the dorsal bundles are attenuated, lance-like, and 
straight (Fig. 3), while in the ventral bundles they present bifid 
tips, provided with a delicate scroll or enveloping hood (Fig. 4), 
except anteriorly, where they are entirely replaced by simple, 
lance-shaped, bent ones (Fig. 5), each with a delicate flange on 
its convex side. The sete of the large fifth somite (Fig. 6) are 
especially modified as a set of about six very stout, golden- 
colored hooks, each with a prominent flange projecting from its 
side near the tip. In the figure some of these hooks are still 
young, and growing up into place beside the perfected ones. In 
addition to these large spines, there are a few delicate, lance- 
shaped ones, as seen in the same figure. 
The shape of the animal’s head is not easily made out, as it is 
drawn back into the first somite upon the slightest disturbance, 
and generally remains there in preserved specimens. In Fig. 7 
its general form, as made out from living specimens, is represented 
somewhat imperfectly. 
The cephalic lobe or head is slightly emarginate anteriorly, 
and bears two delicate antennæ that stand just dorsal to the 
mouth, as a part of the upper lip, and are richly supplied with 
sensory hairs. In the head are also two pairs of black eyes that 
on section appear to have only a simple larval structure. Each 
consists of a minute mass of large, dark-brown pigment granules, 
forming a sort of cup about fifteen microns in diameter, and partly 
surrounding a homogeneous spherical mass of equal size, that 
stains darkly. All this lies some distance from the surface, and 
