MUSEUM OF COMPARATIVE ZOOLOGT. 321 



side of the third segment of the primary (external) flagellum. In full- 

 sized individuals the free portion of the tertiary flagellum may have 

 as many as thirteen or fourteen segments, and the distance between its 

 base and the base of the primary flagellum may contain as many as 

 eight segments. 



The second pair of antennae are about as long as the body. The 

 mandibles (Fig. 18) are two-brauched. One of the branches is terminated 

 by a molar surface (Fig. 1 9), the other by three incisor teeth in the right 

 mandible (Fig. 18), four in the left (Fig. 20). 



The first pair of maxilla? (Fig. 21) consist of two lobes (a, b), which 

 are fringed with setse on their inner borders, and a palpus (r i) bilobed 

 at its free end. 



The second pair of maxillse (Fig. 22) are composed of a bifid inner 

 plate (protognath, a) fringed on its internal border with setse, a middle 

 piece (endognath, r i) composed of one segment, and a large ear-shaped 

 scaphognathite (exognath, r e), whose margin is beset with long and 

 delicate hairs. 



The first pair of maxillipeds (Fig. 23) have a very complicated struc- 

 ture. The inner, prehensile portion consists of two thin, foliaceous lobes 

 (a, b), of which the distal (b 1 ) is the larger. The edges of both are pro- 

 vided with setse. The other parts of the organ are probably tactile and 

 respiratory in their function, and consist of a tongue-shaped middle piece 

 (r i) ; an external, large, oval plate which gives off a long palpiform 

 appendage (r e) ; and two small oval plates (e p, e p') at the base of the 

 larger external plate. The inner margin of the latter, and the distal 

 third of its apj>endage, are fringed with fine seta3. The two smaller ex- 

 ternal plates (e p, e p>') are of a soft consistency, and very probably serve 

 in a measure to aerate the blood. 



The penultimate segment of the endognath of the second pair of max- 

 illipeds (Fig. 24) is produced on the inner side into a large lobe, to the 

 distal margin of which the terminal segment is articulated. The two 

 segments together thus form a broad oval blade, which is reflected upon 

 the inner side of the antecedent segments, and lined with setae on its 

 oral border. The exognath (r e) is long and slender. At the base of 

 the limb, on the external side, are a small roundish epignath (e p) and 

 a small but well-formed gill (b r). 



The third pair of maxillipeds (Fig. 16) are composed of the same 

 parts as the second maxillipeds, but the inner branch (r i) is pediform 

 and twice the length of the external branch (r e). 



The structure of the following thoracic appendages is so well known 



