14 University of California Publications in Zoology [Vol- 23 



both or one only. When chelate or subchelate they are 

 known as chelipeds. In the Brachyura, or true crabs, 

 they are always chelate. When simple they are more 

 properly called ambulatory legs. 



10, 11, 12 and 13. The next four pairs of thoracic appendages 

 are the second, third, fourth, and fifth pairs of legs. 

 They may be either simple, chelate or subchelate, all or 

 in part only. Although usually spoken of as the first, 

 second, third and fourth pairs of ambulatory legs (ex- 

 cept when the first legs, no. 9 above, are simple and 

 included in the total number of ambulatory legs), 

 strictly speaking only the simple pairs are ambulatory 

 in the true sense of the word. In the Brachyura these 

 four pairs of appendages are frequently designated 

 simply as the first, second, third or fourth pairs of legs, 

 in contradistinction to the chelipeds (no. 9, above). 

 This nomenclature, however, has not been adopted in the 

 characterizations of the species listed in this paper. 

 All mention of gills and branchial formulae has been 



purposely omitted as they are more or less difficult for the 



beginner or average student ; only for the specialist have they 



peculiar interest. 

 III. The Abdomen, composed of six true somites, or segments, and a 

 terminal joint, or segment, the telson, a flattened usually 

 pointed tail-piece which never bears appendages and is not 

 considered a true somite. 



14, 15, 16, 17, and 18. The first five pairs of abdominal appen- 

 dages are the pleopods, or "swimmerets, " as they are 

 sometimes called. The first one or two pairs in the males 

 of some forms are variously modified to serve as acces- 

 sory sexual organs ; one or more, or all of the pleopods 

 may be reduced, rudimentary, or wanting. 



19. The last pair of abdominal appendages are the uropods 

 and when present they constitute together with the tel- 

 son the so-called "tail-fan." 



The following seven diagrammatic figures give a graphic exposition 

 of the parts, appendages, and body regions of typical decapod crus- 

 taceans : 



