124 PATTEN AND REDENBAUGH. [Vol. XVI. 



of the carapace, just anterior to the hinge, and passes haemally 

 outside of the branchio-thoracic and longitudinal abdominal 

 muscles to its insertion, close to that of the last-described 

 muscle. These two muscles draw the appendage outward and 

 backward. 



Lastly, a muscle (7^) arising from the haemal side of the 

 carapace passes neurally, and is inserted upon the integument 

 a short distance from the base of the chilarium. 



The Muscles of the Operculum (Text-fig. 5). — These have 

 been minutely described by Lankester, so it will not be neces- 

 sary to give them in detail. The appendages naturally lie 

 flexed backward upon the abdomen. Large extensor, or abduc- 

 tor, muscles {a.b.m.^), arising from the haemal side of the 

 cephalothorax and from the first entapophysis, are inserted 

 upon the anterior lamella and upon the anterior side of the 

 branchial bar {b.c.^) of the operculum. A large flexor, or 

 adductor, muscle (Text-fig. 5 ; Pis. VI and IX, Figs, i and 5, 

 e.b.m}), arising from the haemal side of the abdominal carapace 

 just posterior to the hinge, is inserted upon the posterior lamella 

 and the posterior side of the branchial bar. A small internal 

 branchial muscle (Text-fig. 5, i.bjn.) arises from the neural side 

 of the first abdominal endochondrite (a.e.^) and is inserted upon 

 the inner side of the branchial bar (b.c.^). The branchio-thoracic 

 muscles have already been described. A few strands of mus- 

 cles {p.l.m. and i.l.in.) flex and extend the inner and outer lobes 

 of the appendage. 



The Muscles of the Gills (Text-fig. 6). — These are similar to 

 those of the operculum, except that the extensors or abductors 

 {a.b.mP) arise from the entapophyses of the preceding metamere 

 instead of from the haemal side of the carapace ; and the mus- 

 cles which serve to flex and extend the terminal portions of the 

 appendage are more numerous and better developed. 



4. Digestive System. 



The mouth (Text-fig. 2 ; PI. VIII, Fig. 3, m^ is situated 

 nearly in the center of the neural side of the cephalothorax, and 

 is surrounded by the chelicerae, five pairs of mandibles, and the 



