20 BUSK, ON THE STRUCTURE AND 



in a sort of cauda equina, composed of numerous minute 

 nervous twigs. From the sides of the ventral ganglion, and 

 according to Krohn, from the nervous trunks also, are given 

 off numerous nerves in rapid succession, which, according to 

 the same observer, curve upwards towards the dorsal surface 

 of the trunk, subdividing into numerous twigs, which anasto- 

 mose, and thus constitute a very intricate plexus beneath the 

 integument. The latter part of this statement may perhaps 

 be erroneous, but at any rate there is no doubt of the fact, that 

 numerous small lateral branches are given off, apparently sym- 

 metrically, from the sides of the ganglion itself, as shown in 



The Head. — This portion of the animal is distinctly sepa- 

 rated from the trunk, and is surrounded by a sort of mem- 

 branous hood, which is capable of being drawn backwards 

 over it. The upper surface of this hood is level with that of 

 the trunk, whilst the lower forms a plane inclined from above, 

 and anteriorly downwards and backwards. When fully ex- 

 panded, the hood, except inferiorly where it presents, in the 

 middle line, a longitudinal opening for the mouth, appears to 

 envelop the entire head ; when retracted, the head is exposed, 

 particularly on the sides, when the following parts are dis- 

 played. 1. On each side a series of curved pointed hooks, 

 (fig. 3, c), which, when the hood is expanded, close from either 

 side of the mouth. The number of these hooks does not 

 appear to be very constant, and the anterior hooks are usually 

 shorter than the others. 2. Besides these larger buccal hooks 

 there will be observed, at the anterior extremity of the head, 

 two curved series of smaller denticles (fig. 3, a), one behind 

 the other on either side. 



On each side the inferior surface of the head presents a 

 large, rounded eminence, apparently composed of the muscles 

 by which the buccal hooks are more directly moved, and be- 

 tween these buccal lobes is situated the oval opening in the 

 form of a longitudinal slit or fissure, which is crossed poste- 

 riorly by a kind of fimbriated border, stretching across from 

 one buccal lobe to the other (fig. 3, h). 



The pharynx or oesophagus commencing at this point is a 

 short tube with thick muscular walls, a little larger in 

 diameter than the intestine, and extending but a short distance 

 beyond the junction of the head and trunk. It is bounded on 

 either side by the buccal masses above noticed. On the upper 

 surface of the head, on either side, and pretty close to the 

 median line, will be seen the " eyes," (fig. 7, K). These organs 

 are composed apparently of a mass of black pigment, around 

 the margin of which will be noticed clear points, ox cornew. 



