PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 177 
B. OBSERVATIONS. 
In the account of the morphology of the otocyst, two types will be 
taken for description : — 
(1) Open otocysts containing otoliths (macruran decapods); the 
example will be Palemonetes vulgaris Stimpson. The otocysts of 
the crayfish Cambarus affinis (Say) Girard, and of the prawn Crangon 
vulgaris Say, will be described in only sufficient detail to allow of 
comparison with Palemonetes, and to correct any errors or omissions 
in the descriptions of other investigators. 
(2) Closed otocysts without otoliths (brachyuran decapods) ; the sac 
_ of the green crab, Carcinus menas Lin., will be taken as the example 
of this type. 
For tracing out the development of the macruran otocyst (1), young 
lobsters were used instead of Palemonetes larve, as it is difficult to 
obtain a complete series of the latter, and their small size makes them 
by no means favorable material for studying the embryology of the 
sac. Young lobsters, however, can be had in abundance during the 
hatching season, and are of large size; the otocyst is of the same 
general type as that of Palemonetes. The development of the closed 
otocyst (2) was traced out in the crab for the sake of comparison with 
the macruran type of sac. 
The research represented in this paper was carried on at the sug- 
gestion of Dr. E. L. Mark, to whom I wish here to express my thanks 
for his constant kindness, suggestive direction, and able criticism. I 
am also indebted for valuable supervision and helpful suggestions to 
Dr. G. H. Parker, who directed my work for one year during the 
absence of Dr. Mark. 
1. Material. 
Large numbers of Palzmonetes were obtained from the Charles 
River, Cambridge, at low tide. These river animals live well in either 
salt or fresh water, and may be kept in aquaria without running water 
for an indefinite period. Being so hardy, and at the same time free 
swimmers, they are eminently adapted for intra vitam stains, and 
for physiological experimentation. 
Carcinus mznas was abundant in the soft-shelled condition, at Hadley 
Harbor, Naushon Id., during the months of June and July. The head 
of Great Harbor, Wood’s Hole, was another good collecting ground. 
