I08 PATTEN AND REDENBAUGH. [Vol. XVI. 



A chitinous infolding, or apodeme (Text-fig. 2, apo^, arises 

 from the arthroidal membrane, between the third and fourth 

 joints, and projects into the cavity of the third joint. It fur- 

 nishes attachment for a large flexor muscle arising from the 

 anterior side of the second joint. 



The sixth pair of appendages (Text-fig. 3) are the powerful 

 legs used for burrowing and pushing the animal along through 

 the sand. The inner mandibular portion (Text-fig. 3, vian) 

 lacks gustatory spines, is very massive, and serves as a crush- 

 ing jaw. Two of the sensory knobs of the outer portion of 

 the coxopodite are like the corresponding parts of the other 

 ambulatory legs, but the third or median one is replaced by 

 a spatulate organ, the flabellum (Text-fig. 2)^ flab., and Pis. VI 

 and VII, Figs, i and 2, eiit!"). 



The homologies of these sensory knobs will be discussed later, 

 under the nervous system. 



The fifth joint (Text-fig. 3, 5-pro), instead of forming with 

 the sixth a chela, as in the other legs, is oblong in longitudinal 

 section, and bears upon its distal end a rosette of four shorter 

 spatulate organs, and the sixth joint. The latter (Text-fig. 

 3, 6-dac.) bears at its distal end two small terminal joints, 

 which are opposed to each other and function as a small chela. 



The chilaria (Text-fig. 4), which lie posterior to the mouth, 

 are a pair of small appendages consisting of a single segment. 

 Owen regarded them as detached portions of the sixth pair of 

 appendages, but they undoubtedly represent true appendages 

 and belong to a distinct metamere. 



The first pair of appendages upon the abdomen (Text-fig. 5) 

 are almost completely fused in the median line, and form a large 

 operculum, which overlaps the five pairs of gills. Each half 

 consists of a large proximal portion marked off by radiating 

 creases into triangular areas, and a small distal portion 

 divided into two lobes — an inner lobe, or endopodite (Text-fig. 5, 

 i.l.), consisting of two joints, and an outer lobe, or exopodite 

 {o.l.), consisting of one joint. Two genital papillae upon the 

 posterior side of the proximal portion mark the external open- 

 ings of the oviducts (Text-fig. 5, ov.). There is no gill book 

 upon this appendage. 



