788 On the Homologies of the Crustacean Limb. [October, 



In an Apns lucasanus forty-two millimeters in length there are 

 sixty pairs of legs behind the maxillipedes. There are forty-two 

 segments behind the maxillipedal segment, including the telson, 

 and twenty-seven limb-bearing segments, or sixty pairs of legs to 

 twenty-seven segments, the average being two and six-twenty- 

 sevenths (2,'v) appendages to each leg-bearing segment. On the 

 first eleven leg-bearing arthromeres, or the ten thoracic legs (baeno- 

 meres) together with the first abdominal arthromere, there is but a 

 single pair of appendages to a segment, so that there are forty-nine 

 pairs of abdominal appendages to sixteen arthromeres, or three and 

 one-sixteenth pair of limbs, on the average, to each abdominal 

 arthromere. The fourteenth, fifteenth and sixteenth pairs are 

 situated on two arthomeres, and so on with the succeeding 

 until the limbs become more numerous. On the two arthromeres 

 before the last leg-bearing one, there are twelve pairs of appen- 

 dages, or six to each arthromere. 



This irrelative repetition of arthromeres is only paralleled in 

 one other Branchiate group, the Trilobita. In this group the new 

 segments are interpolated between the head and abdomen at suc- 

 cessive molts, as shown by Barrande. 



The grouping of the body segments into a cephalothorax and 

 abdomen, comparable with those two regions in the Decapoda is 

 but slightly, if at all, indicated in the Phyllopoda. In Limnetis 

 there is no such distinction of regions, in Apus the cephalotho- 

 rax merges insensibly into the abdomen, and it is not until we 

 ascend to the Branchiopodidce that we meet with a well-marked 

 abdomen separated by tolerably clear indications from the 

 thorax. 



The Appendages in general.— The appendages of Crustacea 

 may be divided into four groups : First, the sensory appendages, 

 or antennae, which are in the adult preoral ; second, the organs of 

 prehension of food and of mastication, i. e., the mandibles and 

 accessory jaws, or maxillae and maxillipeds, which are postoral; 

 third, organs of locomotion, whether natatorial or ambulatory, 

 which are appended to the thoracic portion of the body ; and 

 fourth, the appendages of the abdomen, which are both natatorial 

 and concerned in reproduction ; of the latter are the two pairs of 

 gonopoda 1 in the Decapoda, while the eleventh pair of appen- 

 dages in Apus may perhaps be regarded as gonopods. 



1 I have (Amkrican Naturalist, xv, p. 881, 1881) applied the term gonopoda^ 

 (Gr. y,,.;^ generation ; -„.,-, -„,; ( ,- ? foot) to the first and second abdominal lini *> 



