1891.] A Commensal Annelid. 33 
These older larve are, found inside the Annelid’s dwelling, 
sometimes in company with younger larva and even eggs, though 
it is probable that they escape out into the water about this 
stage. In swimming about the provisional sete are brought into 
play as organs of defense, apparently, being thrown out at right 
angles to the body when the animal is disturbed, and trailed 
a ng close to its sides when it swims quietly by means of the 
cilia. These setz are noticably barbed, excessively long, and un- 
like the adult seta ; forming a good illustration of the provisional 
Annelid seta. 
This larva is conspicuous from the metamerically placed dorsal 
pigment blotches, which, it will be noticed, precede the externa] 
appearance of the somites, and are, moreover, represented upon 
the heart by a pair of small black areas near the eyes. . 
The digestive tract now presents three well-marked divisions 
—a mouth and short, ciliated cesophagus leading abruptly into a 
capacious intestine, with some yolk in its walls yet, and opening 
posteriorly into a short rectum that ends at the anus. Here there 
are two papillz with long sensory hairs, such as also occur in a 
tuft upon the median part of the head. 
In sectioning the adult Polydora, eggs in various stages of 
formation are found within the body-cavity. The ovary, in fact, 
appears as a mere mass of modified peritoneal cells, attached to 
and covering over the vascular loops near the nephridia (Fig. 9). 
The youngest ova do not differ perceptibly from the ordinary 
peritoneal cells over the blood-vessels, but they soon enlarge 
and become more and more filled up by accumulating yolk 
globules. In this way there is formed a large botryoidal mass 
of large and small cells (ova), projecting freely into the body- 
_ cavity, and not covered by any membrane. Thus attached to 
blood-vessels, the ova attain a diameter of .06 mm., and then 
break loose into and float freely in the body cavity (Fig. 9.) Here 
they continue to grow till, when apparently ripe, they have a 
r of og mm. In the ovum there is a large nucleus, nearly 
.02 mm. thick, and a very conspicuous nucleolus five microns in 
diameter. This nucleolus is peculiar in having one or more 
rounded elevations or lateral protuberances upon it, which may 
be half as thick as the main body of the nucleolus. 
