SECT. IV THE MUSCULATURE 63 



circular muscles of the ventral parapodium are as 

 inferior in development to those of the dorsal para- 

 podium, as the ventral parapodium itself is inferior 

 in development to the dorsal. They consist of only- 

 two bands. One passes between the ventral cord and 

 the ventral muscle band, to be attached proximally 

 to the ventral membrane of the intestinal sinus, the 

 other is attached direct to the hypodermis at each side 

 of the ventral cord (Fig. 14). The former muscle will 

 be mentioned again in describing the circulation, in 

 which it perhaps plays a more important part than 

 it does in connection with the limbs. 



It is hardly necessary to describe the musculature 

 of the trunk limbs more in detail. That of the head 

 limbs, however, requires special attention, not only on 

 account of the origin of these limbs almost exclusively 

 from ventral parapodia, but also because the masti- 

 catory formula of the Apodidse is, with slight 

 differences, the same as that found in the majority of 

 modern Crustacea, viz., one pair of mandibles and 

 two pairs of maxillae ; although in Apus, the second 

 maxillae are rudimentary. 



Commencing with the mandibles, we there find 

 an arrangement exactly the opposite of that de- 

 scribed in connection with the trunk limbs. In 

 these latter the muscles running into the dorsal 

 branch are the more highly developed ; in the man- 

 dibles, however, the dorsal branch is rudimentary, 

 and the muscles running into the ventral branch 

 are the most developed. The closing muscles radiate 

 from the sinewy mass above described, and are 



