324 The Philippine Journal of Science mi 



of the first 5 somites has a pair of branched structures at- 

 tached to its lower surface, which are known as swimmerets, 

 and the sixth somite has a somewhat similar but larger pair 

 called the uropods. The uropods and the telson together make 

 up the tail fin. 



While zoologists might have no difficulty in identifying palae- 

 mons among a number of other crustaceans more or less related, 

 one who is not specializing in this line might have considerable 

 trouble. For this reason, I shall compare several forms found 

 in the waters about Manila which might be mistaken for them. 

 One of the commonest crustaceans, which may be seen almost 

 any day in our markets, is the "salt water shrimp" belonging 

 to the genus Penaeus. This can be distinguished from the pala- 

 emon by the more or less evident keellike ridge on the dorsal 

 surface of the posterior part of the abdomen, by the absence of 

 large legs, and by the presence of pincers on the first, second, and 

 third legs. 



The pistol crab, belonging to the genus Alpheus, is occa- 

 sionally seen in the markets, and differs from the palaemons 

 in the following particulars : The rostrum is very small or absent, 

 the first legs are strong and provided with pincers, the second 

 legs are weak and have small pincers, and the third, fourth, and 

 fifth legs are weak and without pincers. 



Another form which is used for food by Filipinos, although 

 not highly thought of, belongs to the genus Atya, and is found 

 in mountain streams. It may be distinguished easily from the 

 palaemons by the peculiar pincers of the first and second legs. 

 These pincers are provided with brushes of hairs which are used 

 to catch mud and minute organisms. The third, fourth, and 

 fifth legs have no pincers. 



Finally, I shall mention the large brillantly colored "sea 

 crawfish" or "spiny lobster" (langosta del mar) belonging to 

 the genus Palinurus. It lives along rocky shores washed by 

 the open sea, and is occasionally seen in our markets. All 

 five legs are without pincers, and the antennae are very long 

 and spiny. 



PHILIPPINE SPECIES OF THE GENUS PALAEMON 



Palaemon carcinus Fabricius. Plate I, figs. 1 and la-/. 



The largest palaemon in our collection, and undoubtedly the 

 largest one in the Philippine Islands, I have diagnosed as 

 Palaemon carcinus Fabricius. This conclusion was arrived at 

 after a careful study of a large series, although a comparison 



