230 THE ANATOMY OF LNVERTEBRATED ANIMALS. 



surrounds the gullet and supplies the appendages which lie 

 in front of the operculum. 



The five pairs of appendages which remain resemble the 

 operculum in their general form, and have ascending process- 

 es, which are connected with inward prolongations of the ter- 

 gal wall of the middle division of the body. Their nerves are 

 derived from the ganglia which lie in this region of the body. 



Thus there are altogether thirteen pairs of appendages, 

 eight of which are connected with the anterior, and five with 

 the middle division of the body ; and the appendages in the 

 region of the mouth are essentially ordinary limbs, the basal 

 joints of some of which are so modified as to subserve man- 

 ducation. 



The determination of the homologies of the parts hither- 

 to spoken of as the anterior and middle divisions of the body, 

 and of their appendages, is a matter of some difficulty ; but, 

 on comparing the disposition of the limbs and their nervous 

 supply with what obtains in the higher Crustacea^ it seems 

 hardly doubtful that the first pair of appendages answer to 

 the antennules ; the second, to the antennae ; the third, to 

 the mandibles ; the fourth and fifth, to the maxillae ; and the 

 sixth, seventh, and eighth, to the maxillipedes of Astacus or 

 Homarus ; and, in this case, the anterior division is a ceph- 

 alo-thorax. If the position of the genital openings marks 

 the hinder boundary of the thorax, the middle division of the 

 body represents an abdomen, composed cf five somites. But, 

 on the other hand, it may be that the genital organs open in 

 front of the hinder extremity of the thorax, as in female 

 Podophthahnia^ and that the five somites which form the 

 middle division correspond with the remaining five somites 

 of the thorax of a Podophthalmian. In this case, the region 

 which corresponds with the abdomen in the higher crusta- 

 ceans is undeveloped. 



The alimentary canal of Limuliis is very peculiarly ar- 

 ranged. The gullet passes directly forward and upward, 

 and gradually widens into the stomach, the walls of which 

 are provided with many longitudinal folds. The pylorus is 

 prolonged into a narrow tube which projects into the intes- 

 tine. The two biliary ducts on each side are far apart, and 

 branch out into minute tubules, which form a mass occupying 

 the greater part of the cavity of the body. The rectum, a 

 slender canal with plaited walls, and very short, opens into a 

 sort of cloaca situated between the telson and the sternal wall 

 of the abdomen. 



