334 SUEiiEON F. H. WKLCH ON THE ANATOMY OF 



appear to indicate that not only were these forms in the Shark at 

 an early stage of mature condition of cestoid life, but that these 

 appendages in some forms of the Tetrarhynchidse, althougli pre- 

 sent in the larval condition, are thrown off when the creature 

 reaches the nidus requisite for full development. 



The constricted portion of the parasite colony (the neck) is 

 similar in structure to the other segments ; the body-constituents, 

 the fibrous visceral boundary, and the longitudinal water-vascular 

 canals pass through it to the head, there to be modified to the spe- 

 cial requirements of this part of the parasite. 



The head is composed of two suckers, four proboscides, with 

 four bulbs and adjuncts for extrusion and withdrawal of the pro- 

 boscides situated within an expansion of the visceral boundary 

 continued through the neck and with a stratum of parenchyma 

 between them and the cuticular surface. In the upper half of 

 the head are the suckers and proboscides, in the lower half the 

 bulbs (fig. 1, h). The naked-eye aspect of the fossettes or suckers 

 I have already given ; and to this must be added that the chitinous 

 layer of the skin forms the liuing membrane, and that within this 

 are special layers of muscular fibres for the regulation of the func- 

 tion, these being retained within a fibrous capsule following the 

 contour of the sucker at a distance of -J-jj inch, and separating the 

 special muscles from the general parenchyma of the head (fig, 9, e, 

 andfig. 12, 5). Among the special muscles are radiating fibres 

 which pass from the cuticular lining of the sucker to the capsule 

 (fig. 9, c); and these would by their action induce the function: 

 others pass from one side of the sucker to the other, encircling it 

 both laterally and vertically ; and these would contract the cavity 

 of the hollow and so put a stop to the sucking function, the lateral 

 fibres especially running in a separate layer from the rest (fig. 9, d). 

 Trom the inside of the fibrous capsule of the sucker diverging 

 muscular fibres pass into the general parenchyma of the head ; 

 and these would render the capsule firm for the special radiating 

 fibres to act from (fig. 12, c). 



Pig. 11 illustrates an exserted proboscis, magnified, and fig. 12 

 one all but retracted, a slight eversion of the booklets at the 

 base being present. The exserted proboscis is short, thick-set, 

 club-shaped, somewhat broader near the free end than at the 

 base, yf^ inch long by y|^ inch thick. It is closely studded 

 with booklets with points directed downwards ; the number and 

 arrangement of them it is iuipos^iblc to determine accurately, from 



