294 M. Krohn on the Anatomy and 



round the base of the hooks outside and behind, in proceeding 

 toward the lower surface of the head, behind the mouth. As we 

 have already said, the hood only covers a portion of the head, 

 leaving the lower surface almost completely exposed ; the result 

 is, that its lateral parts must be wider than its upper and lower 

 portions. Between the two leaflets composing it we observe very 

 evident slender fascicles of fibres, which have a parallel course to 

 the outline ; these fibres probably serve for the expansion of this 

 part, whilst a simple mechanical action, the straightening of the 

 hooks, and the swelling of the head, which is the consequence of 

 it, casts back this organ. 



Hooks. — These are composed of homy fibres, excessively de- 

 licate, and having a longitudinal direction. Their base is hollow, 

 and contains a substance which becomes white and turbid in 

 spirits of wine ; it is probably a kind of bulb destined to rege- 

 nerate the hook when this latter is worn or destroyed. 



Muscular apparatus. — The principal mass of the head is com- 

 posed of muscles arranged symmetrically on the two sides of 

 the head; the most voluminous among them are those which 

 form its base, and on this account, these masses, seen in profile, 

 appear to be obliquely truncated ; in fact, each of them consti- 

 tutes the corresponding half of the head. When they are in 

 action, raising the hooks, they form two considerable eminences 

 which project beyond the two prominences which surround the 

 mouth. They are resolved into numerous fascicles, the direction 

 of which it is difficult to detect. We know however positively 

 that most of these fascicles are inserted at the base of the hooks, 

 and others on to a thin and hard plate, which I must here de- 

 scribe. This plate exists on each side, immediately beneath the 

 skin. It extends on the surface of the muscles in question, at 

 first between the bases of the hooks and the points of insertion 

 of the hood ; it then forms a curve, following this insertion as far 

 as the anterior margin of the head; and lastly, growing thinner, 

 enters into the eminences which bear the little spines above- 

 mentioned. 



With regard to the other pairs of muscles, I confess that I 

 have not been able to follow them in a satisfactory manner ; 

 their minuteness bafiled my patience ; and if I were able to indi- 

 cate the origin and insertion of some of them, I should still find 

 it impossible to explain their action. 



Pharynx. — The pharynx is a short duct, situated in the mid- 

 dle of the head, and only a little wider than the stomachal swell- 

 ing. Laterally, it is limited by the two muscles of the hooks, 

 and, as it seems also, by walls furnished with very decided mus- 

 cular fibres, crossing one another. 



II. Trunk. — The cavity of the trunk is filled, during the life 



