442 Prof. K. Grobben on the Genealogy and 



number (20)* of the segments of the body as compared with 

 what we find in Apus and Estlieria^ the sharp separation 

 betvA'een llie tlioracic and the abdominal region, the latter of 

 which is devoid of appendages in the adult, and the modifica- 

 tion of the second antenna into a lamelliform structure in the 

 female and into an apparatus of considerable size in the male. 

 By the absence of the shell, the elongated form of the body, 

 the powerful development of the musculature of the trunk, 

 "which comes into action in the darting motion of the body, 

 and the not very large number of segments in the thorax and 

 abdomen, BrancJiijms is characterized among the Euphyllo- 

 poda as the form best adapted to the movement of swimming. 



On comparing Aims with Branchipus the first point which 

 will have to be noticed as a primitive character in the case of 

 the former is the larger number (33) t of the segments of the 

 body. In Apus, too, the transition from the thorax to the 

 limbless abdomen appears to be a more gradual one, owing to 

 the fact that there is a continuous and very striking diminu- 

 tion in the size of the thoracic appendages towards the rear. 

 As a peculiarity which is found in Apus alone among the 

 Euphyllopoda must be mentioned the large number (63) of 

 the thoracic appendages ; the eleven anterior segments of the 

 body each bear one pair of appendages (they correspond to 

 the eleven limb-bearing thoracic segments of Branchipus), 

 while upon the following seventeen segments we find a larger 

 and posteriorly increasing number of limbs, so that to the 

 last two limb-bearing segments there together belong twelve 

 pairs of appendages. I will not here attempt to decide whether 

 in the posterior section of the thorax of Apus we have to 

 deal with a fusion of several segments to form larger annuli, 

 or with a multiplication of the appendages within the 

 segments, although I rather incline to the latter view. In 

 any case, whether concentration of segments or multiplication 

 of the appendages has taken place, it would be a question of 

 a secondary condition. 



Thus, should the actual number of the body-segments be 

 determined by the number of the appendages, the segmenta- 

 tion of the body, in this case unusually extensive, would 

 surely have to be regarded as a secondary character. 



Ajpus bears a small shield-sliaped shell covering the ante- 

 rior segments of the thorax, and herein possesses, as opposed 

 to Branchipus, an old character belonging to the common 



* Among the Brancliipodidae the genus Poh/artemia alone possesses a 

 larger number of segments (namely 22), nineteen of which bear appen- 

 dages. 



t The numbers refer to Apus cancriformis. 



