18 GILBERT C. BOURNE. 



tioiis has been carefully studied, and I take this opportunity 

 of giving a drawing (fig. 15) of three fibres of the extrinsic 

 muscles attaching the anterior end of the odontophore to the 

 integuments of the snout. These wei'e specially well-stained, 

 and it is obvious that tlie ends of the filjres nearest the snout 

 are in a state of contraction, while their odontophoral ends — 

 the lower ends in the figure — are relaxed. The fibres are not 

 round but elongate oval in cross-section. 1'hat on the right 

 has been cut through its long axis; in the two fibres on the 

 left the section passes through the shorter axis, near the edge 

 of the fibres. It can be seen that each fibre is a single 

 metamorphosed cell, with a single nucleus situated near its 

 broader end. The central poi-tion of the cell, in which lies 

 the nucleus, is composed of but little-altered cytoplasm, 

 exhibiting an alveolar or reticular structure, differing from 

 the normal only in the fact that the meshes of the reticulum 

 are very regularly disposed in rectangular fashion. This 

 cytoplasmic core of the fibre is invested by a sheath of con- 

 tractile substance, which is thickest at the two ends of the 

 long axis of the oval, and therefore appears as two bands in 

 the right-hand fibre in the figure, while in the two left-hand 

 fibres only the contractile substance is cut through. The 

 whole is invested by a delicate sarcolenima. The most 

 interesting thing about these fibres is that the reticular 

 arrangement of the cytoplasmic core corresponds exactly with 

 the striations of the contractile substance in the upper part of 

 the fibre on the right side of the figure, and in the left-hand 

 fibre the cross-striations are very obvious and close together 

 in the uppermost contracted part of the fibre, but lower down 

 as the fibre becomes more relaxed, the dark transverse lines 

 become progressively broader and faintei", and each may be 

 seen to be made up of a number of dark longitudinal stride, 

 which may well be interpreted as nodal thickenings of a 

 reticulum. It is, of coui'se, possible that the difference 

 between the two ends of the fibres is due, not to a difference 

 in the state of contraction, but to a greater specialisation of 

 the broader end. Whichever interpretation is correct, the 



