NEW RHABDOCCELE COMMENSAL WITH MODIOLUS 381 
This would appear to be inharmon}^ with its natural surroundings. 
If its instinct led it to make long excursions in a straight line the 
chances of its being carried away from its host would be much 
greater than they must be with its actual hesitating manner of 
progression. 
The test for the reaction to light was repeated with the same 
general result. That is, they tend to move away from the light. 
This instinct, again, it would appear, is what should be expected 
of a worm living as a conmiensal within the mantle-cavity of a 
mussel. 
The ciliated young within the embryonic capsule are very active. 
They are constantly swimming about, and, on account of the con- 
fined space in which a pair is obliged to move, the result of the 
ciliary action alone would be to drive them round and round. 
Beside the ciliary motion there are frequent changes of direction 
effected by turning the anterior end of the body to one side or the 
other. Indeed the characteristic habits of motion of the adult 
can be detected in the young before they have escaped from the 
capsule. When the young break through the capsular wall they 
wander freely in the mesenchjmie. Thus they were observed 
in all parts of the body pushing their way industriously, but never 
continuing in a straight course for long. They were seen even 
lying between and in front of the eyes, and indeed in every part 
of the body, but most of the fully developed young are found in 
the posterior third of the body of the mother. When the fully 
developed young escape from the body of the mother their be- 
havior in the sea water is essentially like that of the adults. Many 
free individuals were collected which were but little further ad- 
vanced in development than some which were still in the embry- 
onic capsule. On one occasion a young worm was observed push- 
ing its way through the dense median mesenchyme. When it 
reached the edge of the denser portion it pushed for an instant 
as if it had encountered something which was resisting itsprogress. 
The obstruction gave way suddenly and the worm passed quickly 
into the thinner mesenchyme which lay along the anterior margin. 
It is probable that the worm had strayed into the lumen of the in- 
testine and was seen forcing its way out through the intestinal wall. 
