TWO PARASITIC FORMS OF THE FAMILY TETBAEHYNCHlDiE. 335 



the close apposition of one to the other ; there cannot, however, 

 be less than one thousand on each proboscis ; and tlie rows of them 

 appear to wind spirally to the top. When tlie proboscis is re- 

 tracted, the tips of the booklets point upwards and converge 

 towards each other in the centre of the hollow cylinder then formed. 

 This will be apparent on comparing fig. 12 with 11, and on re- 

 membering that the evolution of the proboscis is similar to the 

 drawing-out of the inverted finger of a glove ; the simile will 

 render lucid also the process of boring into the tissues, and how 

 in the gradual unfolding of the armed projectile the rows ot 

 booklets come into play, those near the base being firmly fixed 

 before the coming into action of those nearer the tip. The 

 booklets diff'er somewhat in shape and size according to situa- 

 tion upon the proboscis, whether towards the base or near the 

 free end ; if the former, then the characters are seen in fig. 10 

 but towards the free end the claw is lengthened out and straighter 

 clearly for the purpose of giving these a wider range of action 

 their average length is about ^wo inch. Their tip is very sharp, 

 the limb is curved, the base is extended out and flat ; the struc 

 ture is transparent, and apparently consists of a very dense outer 

 sheath and a soft granular internal core ; the base is firmly at- 

 tached to the surface of the proboscis. I am inclined to consider, 

 from the facts of acids having no influence on them, and prolonged 

 immersion in strong liquor potassae and glycerine rendering them 

 soft and pliable, that the composition of the booklets is chi- 

 tinous, similar to the claws of rapacious birds. Eig. 12 shows 

 the proboscis to be muscular in structure, contained within a spe- 

 cial thick sheath separating it from the general parenchyma of 

 the head. Immediately within the sheath is seen a layer of mus- 

 cular fibre forming an outer cylinder (fig. 12, e), the fibres directed 

 vertically ; these appear to arise low down from the inside of 

 the sheath, and, passing upwards to the base of the proboscis, 

 curve (in the inverted condition) inwards and downwards, but 

 in the exserted state are continued up, forming the outer stratum 

 to which the base of the booklets is attached, and eventually 

 merging into the strong circular muscle for the retraction of 

 the proboscis (fig. 12, ff). Commingled with this outer layer, 

 where it is in contact with the booklets (i. e. where it helps 

 to form the proboscis), is a circular layer of fibres entirely limited 

 to the proboscis (fig. 12, f), while from the most inverted end of 

 the proboscis (or forming the core of it when exserted) a strong 



