PEENTISS: 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 Palsemonetes 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 Palaemonetes, 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 mtenas 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 Palaemonetes larvae, 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 Palaemonetes. 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 

 fur 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 Palaemonetes 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 maenas was abundant in the soft-shelled condition, at Hadley 

 Harbor, Xaushon Id., during the months of June and July. The head 

 of Great Harbor, Wood's Hole, was another good collecting ground. 



