THE MEROSTOMATA. 229 



The mouth is placed in the centre of the sternal surface of 

 the anterior division ; the anus opens on the same surface, at 

 the junction between the middle division and the telson. A 

 movable, escutcheon-shaped labrum projects backward in the 

 middle line, immediately behind the subfrontal area (d) ; and 

 on each side of it is a three-jointed appendage, the second joint 

 of which is prolonged in such a manner as to form with the 

 third a pincer or chela. The attachment of this appendage is 

 completely in front of the labrum, which separates it from the 

 mouth. 



In each of the next five pairs of appendages, the basal 

 joint is enlarged ; and, in the anterior four, its inner edge is 

 beset with numerous movable spines. The attachment of the 

 basal joint of the foremost of these appendages (the second 

 of the w^hole series) is in front of the mouth ; but its pro- 

 longed, spinose, posterior and internal angle may be made to 

 project a little into the oral cavity. The basal joints of the 

 following three appendages are articulated at the sides of the 

 mouth, and the inner angle of each is provided with a spinose 

 process which projects into the oral cavity. The second, 

 third, fourth, and fifth appendages in the females are chelate ; 

 in the males of most species the second, and sometimes the 

 third, are not chelate. The large basal joint of the sixth ap- 

 pendage is almost devoid of spines, and bears a ctirved, spat- 

 ulate process, which is directed backward between the ante- 

 rior and middle divisions of the body. The fifth joint of this 

 limb carries four oval lamellae. The appendages which form 

 the seventh pair, very unlike the rest, are short, stout, and 

 single-jointed. 



The eighth pair of appendages, again, are of a totally dif- 

 ferent character from those which precede them. They are 

 united in the middle line into a single broad plate, which 

 forms a sort of cover, or 02?erculum^ over the succeeding ap- 

 pendages, when the animal is viewed from the sternal side. 

 On the dorsal face of this plate are seated the two apertures 

 of the reproductive organs. 



From the inner face of the anterior, or sternal, wall of 

 each half of the operculum a strong process arises, and passes 

 upward to be attached to a corresponding process of the ter- 

 gal wall of the anterior division of the body. By far the 

 greater part of the large levator muscle of the appendage 

 arises from the tergal wall of the anterior division of the 

 body, and the nerve which supplies the limb is derived direct- 

 ly from the posterior part of the multiganglionate cord which 



