MUSEUM OF COMPAKATTVE ZOOLOGY. 311 



baling the water out from the branchial chamber before the least trace 

 of the gills has appeared. As the currents of blood in the zoea stream 

 along the sides of the carapace, they are aerated through the delicate 

 integument by the constant flow of water kept up by the movement of 

 the scaphognathite, which thus plays the same part as in the gill- 

 bearing adult. 



Next follow the three pairs of maxillipeds, which, in the early stages 

 of development, serve as swimming-organs. The first pair is the smallest. 

 Its basal segment (PI. I. Fig. 17, VI) has a strongly convex inner 

 margin, which is beset with setse. This part of the limb acts as a jaw, 

 while the rest of the limb is used for locomotion. In this the appendage 

 recalls the structure of the limbs in the Merostomata. The inner branch 

 is composed of one segment. The outer branch (PI. I. Fig. 14) is 

 obscurely divided into two segments, and is considerably longer than the 

 inner branch. 



The second pair of maxillipeds (PI. I. Fig. 14, VII) consist in like 

 manner of a basal segment and two branches. The inner branch con- 

 sists of three segments, of which the proximal and middle are of about 

 equal length, the distal very short. The outer branch is longer than 

 the inner, and is indistinctly divided into five or six segments. Of 

 these the first and second are of equal length, and form together the 

 bulk of the branch. The following segments are crowded together near 

 the tip of the branch, and the lines of division between them can barely 

 be discerned. 



The third pair of maxillipeds (PI. I. Fig. 1 4, VIII) exceed the second 

 pair in length, but have essentially the same structure. The second 

 pair, again, are longer than the first pair. The branches of all the max- 

 illipeds are furnished with long setse. 



Behind the third pair of maxillipeds are seen the rudiments of the 

 two following pairs of thoracic appendages (PL I. Figs. 14, 17, IX, X ; 

 Fig. 13). They are two-branched, each branch having the form of a 

 simple sac, without trace of a division into segments.* 



There are as yet no traces of abdominal appendages.t 



* Du Cane (Ann. Nat. Hist., Vol. II. PI. VI. Fig. 2. 1839) and Bobretzky (op. tit., 

 p. 204, PI. V. Fig. 17) both represent the first larval stage of Palcemon with the 

 rudiments of the first three pairs of walking-feet in the form of simple sacs. I am 

 inclined to believe that a more careful dissection would have revealed, in both cases, 

 the structure which I have found in Palcemonetes vulgaris, viz. two pairs of double 

 sacs. 



t According to Kroyer (Monografisk Fremstilling af Slsegten Hippolyte's nordiske 



