A NEW SPECIES OF PENT ASTOM UM. 173 



to assert that this was generally the case, without an examination of fresh 

 specimens, but it receives corroboration from Leuckart's account of P. probos- 

 cideum. Some of these oblique fibres, on reaching the inner surface of this 

 epithelium, turn and pass along it, thus assisting in the formation of the trans- 

 verse muscular coat already described (fig. 16). 



Before quitting the muscular system, three points require a brief considera- 

 tion, — the modifications which it undergoes in the tail and in the head, and 

 its disposition with respect to the cephalic hooks. 



In the tail about the point at which the longitudinal fibres first became 

 noticeable, the two lamellae have a somewhat different disposition ; instead of 

 being inclined to each other at a considerable angle, they are much more nearly 

 parallel, but separating from each other to give place to the intestine, they 

 again approach slightly as they are inserted into the body-wall dorsally (PI. 

 XXVII. fig. 13). 



The second point may be dismissed in a very few words ; on approaching 

 the anterior extremity of the body, the splitting of the layer of oblique fibres 

 towards its ventral margin is much further carried out (PI. XXVII. fig. 10, m.o), 

 and the bundles of longitudinal muscle- fibres in relation with it are more nume- 

 rous and larger, so that the ventral band of muscles is much more prominent. 

 Concurrently with this change, but proceeding much more slowly, is a gradual 

 diminution of the bundles of longitudinal fibres in the remaining portions of the 

 body. As we reach points still more anterior in the animal (about on a level 

 with the oesophagus) the ventral longitudinal bands undergo a similar diminu- 

 tion, until in the neighbourhood of the hooks the regular longitudinal muscle- 

 coat is represented only by a few isolated fibres. 



The oblique fibres, on the contrary, become more numerous, and are dis- 

 posed more nearly parallel to the median plane of the body, until finally they 

 come into relation with the hooks, and there become adapted to a special 

 function, as will presently be seen. 



The Muscles of the Hooks. — The muscles which act upon the hooks have 

 been studied by means of almost complete series of sections, both longitudinal 

 and transverse, and though the general result of this inquiry is to corroborate 

 Leuckart's # account of the matter, there are one or two differences which 

 require to be noticed. 



Attached to the hook itself are four muscles, one to the dorsal and three 

 to the ventral extremity of its base. 



The Extensor unci (PI. XXVII. fig. 12, u.e), as the first of the muscles 

 above mentioned may conveniently be called, arises from the inner or ventral 

 surface of the basal joint of the hook, and its fibres pass forwards and inwards, 



* Loc. tit., pp. 46, 47. 

 VOL. XXXII. PART I. 2 F 



