14 W. BALDWIN SPENCER. 



attached to the hook itself ; (2) those attached to the basal 

 joint and basal chitinous thickening. In the following descrip- 

 tion the letters correspond to those used to distinguish the 

 muscles in fig. 39. 



(1) Muscles attached to the Hook itself. — These are 

 only two in number. 



(a) Extensor of the Hook. — This forms a fan-shaped 

 mass entirely enclosed within the sheath. It arises from the 

 upper and posterior internal surface of this, and its fibres 

 converge as they run forwards, and have a strong tendinous 

 attachment to the upper angle of the hook. 



{b) Flexorofthe Hook. — This arises from the lower and 

 posterior internal surface of the sheath, and passes forwards as 

 a strong, stout muscle to be attached to the lower angle of the 

 base of the hook. This must correspond to the two muscles 

 described by Stiles as m. flexor interior et exterior; but 

 in this form I can only find a single large muscle, which has 

 the same relationship to the lower angle of the hook which 

 the extensor has to the upper angle, except that in the case 

 of the former the attachment to the hook is broad and not 

 tendinous. 



(2) Muscles attached to the Basal Joint and Basal 

 Chitinous Thickening. — These are four in number. 



(c) Internal and External Protractors. — This, when 

 viewed from above, has a fan shape, and must be equivalent to 

 the m. protractor externus et internus of Stiles, and 

 most probably to that described by Macalister^ as present in 

 P. imperatoris, where this author states that "each hook 

 has also a fan-shaped depressor muscle, which is only a 

 specialised part of the longitudinal muscular layer of the 

 body-wall." The muscle in question is a large one arising 

 from the internal, external, and anterior (with regard to the 

 median line of the body) surfaces of the basal joint of the 

 hook. The fibres spread outwards and slightly downwards to 

 be inserted into the body-wall. The action of this muscle is 

 to pull the whole hook apparatus downwards and outwards, so 

 • Loc. cit., p. 63. 



