98 COMPARATIVE ANATOMY 



called the oostegopod. In it the gnathobase and four succeed- 

 ing endites are normal, but the distal endite is greatly enlarged 

 and is confluent with the sub-apical lobe. The latter structure 

 is expanded and modified so as to form a circular shallow cup 

 upon which the flabellum, similarly expanded into a pedun- 

 culated circular plate, fits like a lid. The edge of the flabellum 

 posterior to its peduncle is notched to form an aperture leading 

 into the cup. The bract is reduced to a small digitiform 

 appendage (fig. 22, XVI}. The posterior thoracic limbs 

 (3-10) are very similar to the first postgenital limb, which 

 has been taken as a type. In the more anterior members of 

 the series the axis becomes progressively longer, the endites are 

 carried further apart, the dorsal lobe of the sixth endite becomes 

 more prominent, the sub-apical lobe, though still prominent, 

 becomes reduced in size, and the flabellum is elongated in the 

 direction of the axis. The second and first thoracic limbs 

 present special features. 



In the second limb, the axis or corm is divided by a soft 

 fold of the cuticle into a proximal and a distal segment. 

 The gnathobase and second endite are borne on the proximal 

 segment, the four remaining endites and the bract and flabellum 

 on the distal segment. The gnathobase presents no peculiar 

 features, but the remaining endites are narrowed and elongated ; 

 the sub-apical lobe is very small, and the bract and flabellum 

 are reduced in size relatively to the other parts. The peculiar 

 hinge-like attachment of the sixth endite should be noticed. 



The first thoracic limb is modified in a very striking manner. 

 The axis is divided into four distinct segments. Of these, 

 the most proximal bears the gnathobase. The next segment 

 bears the second endite, modified to form a short filamentous 

 process, divided into rings or joints by as many annulations 

 of the chitinous cuticle. The third segment of the corm bears 

 the third endite, similarly modified to form a filamentous 

 process with about forty annulations. The distal segment 

 of the corm bears the fourth, fifth, and sixth endites. The 

 fourth is an elongate filament, with about fifty annulations ; 

 the fifth, situated at the extremity of the axis, is very long 

 and slender, and is divided into about eighty annulations. 

 The sixth endite is reduced to a small boat-shaped process, 

 the proximal end of which fits into a notch in the axis formed 

 by the very much reduced sub-apical lobe. The bract 



