MUSEUM OF COMPARATIVE ZOOLOGY. 263 



III. Pinnixa. 



As long ago as 1835 the zoea of Pinnotheres was raised from the egg and 

 figured by J. V. Thompson* Although this was one of the earliest known 

 cases of metamorphosis among the Crustacea, Thompson's memoir has remained 

 the sole contribution to the developmental history of the Pinnoiheridce up to 

 the present time. 



In the latter part of August, 1878, a large zoea was not uncommon in Narra- 

 gansett Bay, which developed in the aquarium into a species of Pinnixa. 

 The development of this species is remarkable for the complete suppression of 

 the megalopa-stage, the zoea figured on Plate IV. (Figs. 5-15) passing directly 

 into the first stage of the crab, represented on Plate V. 



In the last zoea-stage (Plate IV. Figs. 5-15) the carapace is armed with 

 long frontal, dorsal, and lateral spines. The natatory feet are very short in 

 proportion to the size of the body, the inner branch of the second pair (Fig. 11, 

 a) rudimentary, as usual with Brachyuran zoese. The third pair of maxillipeds 

 and the five following pairs of thoracic feet are folded under the carapace. 

 On dissecting them out from the body, they are seen to be quite perfectly 

 formed, as shown by Fig. 7. The third maxillipeds (Fig. 8) possess their 

 three constituent parts (a, b, c) and a gill (d) at their base. 



The abdomen (Fig. 10) is short, its penultimate segment expanded laterally 

 into two lobes, which extend backward on each side of the terminal segment.t 

 The terminal segment is ovate, produced posteriorly into a long spine on either 

 side. Between the lateral spines and a small median sinus are three short, 

 finely feathered setae on either side. 



There are four pairs of simple abdominal appendages. 



There is a black pigment spot at the base of the dorsal spine, one near the 

 base of each of the lateral spines, and several on the abdominal somites. 



From tip of frontal spine to posterior border of carapace, 2.5 mm. From 

 tip to tip of lateral spines, 2 mm. Length of frontal spine, 1 mm. 



In the first crab-stage (Plate V.), which immediately follows the last zoea- 

 stage just described, the carapace measures less than 1 mm. in length by 1.5 mm. 



* Memoir on the Metamorphosis and Natural History of the Pinnotheres or Pea- 

 Crabs. By W. [J. V.] Thompson, F. L. S. Entomological Magazine, Vol. III. pp. 

 85 - 90. Figs. 1, 2. April, 1835. 



t This peculiarity is also seen in the undetermined zoea, figured by Claus, " Un- 

 tersuchungen," etc. Taf. XIV. Figs. 1, 2 (p. 65). The abdomen of the zoea of 

 Pinnotheres pisum apparently has a like form, judging from Thompson's rude figures 

 (I. c), in which the penultimate and ultimate segments are represented as a single 

 piece. 



In " Fur Darwin" Fritz Miiller figures the tail of a Pinnotheres (Fig. 19). It has 

 the shape generally seen in Brachyuran zoeae, while Fig. 22, "of unknown origin," 

 represents a tail like that of Pinnixa and Thompson's Pinnotheres. Is there no con- 

 fusion here ? 



