CALIGOIDEA. 1337 



one either side, to each of the natatory legs, the inner passing also to 

 the abdomen; while in Caligus, the ganglion is not furcated, but 

 instead, narrows and terminates in two pairs of nerves for the two 

 anterior pairs of natatories, and a central nerve, which affords branches 

 for the third pair (or apron), and then furcating, gives a branch to 

 the fourth pair, and extends into the abdomen. 



In Argulus, there is a single ganglion behind the mouth, as in Cali- 

 gus ; but there are faint traces of a division into rings, each giving out 

 a pair of nerves. They indicate the actual composite character of 

 the ganglion in Caligus, although there is no appearance of such a 

 subdivision. 



Organs of Digestion. — The alimentary canal in Caligus (fig. 1 h) is 

 composed of three distinct parts, corresponding to the oesophagus, the 

 stomach, and the intestine. 



The oesophagus constitutes one-sixth the whole length of the ali- 

 mentary canal, and in large individuals is about one-sixteenth of an 

 inch long. It extends in the form of a long slender tube, of uniform 

 diameter, to the stomach, and passes a short distance into its cavity. 

 Its insertion in the buccal mass may be seen in fig. %) 5 , which is an 

 under view of this organ. The anterior opening is closed by two 

 fleshy folds, which have already been described when speaking of the 

 organs and muscles of the buccal mass. At its commencement, there 

 is an oblong enlargement (fig. I), longitudinally striated, which may be 

 considered a pharynx. The communication with the stomach is 

 closed, but whether by a sphincter or valve is undetermined. The 

 peristaltic motion frequently seen in the stomach and intestine, never 

 extends into the oesophagus. 



This portion of the alimentary canal is readily separated into two 

 membranes. The inner, the mucous coat, is thin and transparent, 

 and very smooth. The outer is much thicker and scarcely semi-trans- 

 parent; its muscular fibres were not distinguished. When highly 

 magnified, its exterior surface appears very uneven. If the mouth is 

 detached from the body with care, the oesophagus often continues 

 attached to it, and presents the appearance exhibited in fig. p s . The 

 inner coat is usually entire to its termination in the stomach, while 

 the outer, which is continuous with the exterior membrane of the sto- 



335 



