314 BULLETIN OF THE 



become divided by a transverse suture near its base, so that it now 

 consists of two segments, a short proximal and a long, slender, distal 

 one. The external branch of the second antennae (PI. II. Fig. 5, //, 

 r e) has increased in size, but otherwise has suffered little modification. 



The posterior border of the labrum, which bounds the mouth in 

 front (PI. II. Fig. 7), when viewed under a high magnifying power is 

 seen to be serrate with minute teeth. 



The mandibles and maxillae (PL II. Figs. 5, 8, 9) have undergone little 

 change. 



A fifth pair of swimming-feet (PL II. Fig. 5, X) has developed from 

 the anterior of the sac-like buds behind the last pair of swimming-feet 

 of the preceding stage. This pair of swimming feet corresponds to the 

 second or larger pair of chelipeds of the adult Palcemonetes. They are 

 similar in make to the preceding pairs, but somewhat shorter. The 

 antecedent pair of swimming-feet have gained in length, so that they 

 now equal, if they do not slightly exceed, the pair in front of them. 

 The segmentation of the inner branches of all the swimming-feet is more 

 distinct than at first, dividing them into four segments, at least in the 

 hindmost three pairs. 



A new pair of sac-like appendages (PI. II. Fig. 5, XII 1) have devel- 

 oped behind the swimming-feet, so that we still have two pairs (XI and 

 XIII), which will disclose swimming-feet after the next moult. The 

 gills have begun to develop at the base of the swimming-feet. 



The broad triangular fin which terminated the abdomen in the last 

 stage has by exuviation freed the posterior pair of abdominal append- 

 ages (PI. II. Fig. 5, XIX), of which we have seen the rudiments 

 through the transparent integument in the previous stage. Both 

 branches of these appendages are developed as two oval plates, of which 

 the outer is two thirds the length of the telson, the inner about one 

 third. The larger plate is fringed with fine setae on its inner, and on 

 the hinder portion of its outer, border. The telson itself, as before said, 

 is much narrower than before, and is separated by a suture from the 

 sixth abdominal segment. Its hind margin now bears but twelve long 

 setae, the external one on each side having disappeared. 



On the lower side of the thorax, between the bases of the swimming- 

 feet, is a good deal of green pigment beautifully arranged in dendritic 

 figures. Other parts of the body also are rendered conspicuous by spots 

 of red, green, and yellow. 



Length, 3.5 mm. 



Fourth Larval Stage (PL II. Figs. 10-15; PL III. Figs. 1 - 3). —At 



