No. 457.] CHA TOPTERUS PERGAMENTACEUS 39 
because of the presence, in nearly every instance, of male and 
female in the same tube. I also found tubes containing males 
only (4) or females only; among the latter the eggs on seven 
females (six Polyonyx and one Pinnotheres) thus taken were in 
various stages of development, while two other females had, just 
a short time before capture, liberated their broods. The mature 
male of Pinnotheres is small enough to pass into and out of the 
orifices of the tubes, but how or when the eggs of the isolated 
individuals of Polyonyx are fertilized remains an open question. 
The breeding season of the two more common commensals, 
Polyonyx and Pinnixa, extends through the whole summer. 
Females with eggs in all stages of development, together with 
females which had just liberated their broods, were taken in my 
first collection, June 21st (1904), and even in my last collection, 
October 25th, every full-grown female taken bore developing 
eggs, or had very recently liberated her brood as shown by the 
few imperfect larvæ clinging to the pleopods, or by the position 
of the abdomen, or “apron,” not being closely appressed to the 
body. At the time of the last collection the breeding season of 
. Pinnixa must have been near its close but that of Polyonyx con- 
tinued unabated. 
The larvae do not long remain in the tubes after they are 
hatched but soon find their way out and they may then be taken 
in the tow in considerable numbers from June till November. 
Young crabs, not over two or three millimeters in width, are 
found in the tubes during the middle and latter parts of the 
breeding season, but they are frequently overlooked because of 
their tendency to press between the appendages or the folds of 
. Chaetopterus and hide there. In this manner four young crabs, 
about two millimeters wide, were overlooked when four tubes 
were opened, and from each the worm together with two full- 
grown commensals were removed, and only after the worms were 
submerged in the killing-fluid did the activity of the crabs make 
their presence known. Once in the tubes it is quite probable 
that the crabs remain there and are later prevented, through 
their own growth, from escaping. 
The act of moulting was not observed in these commensals 
but recently moulted individuals of Polyonyx were taken on 
