TREMATODA 



43 



some time after the embryo has escaped, one may notice 

 elongated masses of sarcode formed by the coalescence of the 

 globules. Whilst still in the egg, one end of the primitive 

 embryonal mass becomes gradually narrowed, cilia at the same 

 time appearing. This part becomes the future head, eventually 

 acquiring the form of a cowl. Whatever form the body of the 

 embryo may display after extrusion from the shell, the head 

 retains its conical shape, the cone itself being narrowed 

 or widened only when the larva is subjected to abnormal 

 conditions (fig. 14). Whilst the head is undergoing develop- 

 ment within the shell, one, two, or sometimes three, pyri- 

 form masses make their appearance within the cone ; and 

 after the embryo has escaped, these structures become more 

 marked (fig. 10). The sarcode-globules refract light strongly; 

 and, when the larva is not compressed in any way, they move 

 freely within the somatic cavity. In well- 

 developed embryos, whilst still in the egg, 

 the cilia are observed to clothe every part 

 of the larva except the oral papilla. This 

 minute nipple-like projection measures 

 about the 5 of an inch transversely, form- 

 ing a very simple kind of unarmed pro- 

 boscis. When the head of the free embryo 

 is viewed from above, the proboscis looks 

 like a central ring surrounded by a series 

 of regular folds, which radiate outwards 

 like the spokes of a wheel. The ridges 

 thus formed support numerous cilia, these 

 latter projecting at the circumferential mar- 

 gin of the cephalic cone in such a way as to 

 present the figure of a star. Dr Harley 

 has admirably represented this character, 

 which is shared by many other parasitic 

 larvaB. Throughout the greater part of the time, whilst the 

 embryo is still resident within the egg, the broad neck or base 

 of the cephalic cone forms a fixed point of resistance by its firm 

 attachment to the inner wall of the shell; and this structural 

 union, so long as it remains intact, enables the embryo to move 

 not only its head and body from side to side synchronously, but 

 also each part independently. When the time for final escape 

 is drawing near, the vigorous movements of the cone-shaped 

 head seem chiefly concerned in loosening the membranous 



FIG. 12. Egg of Bilharzia, 

 with contained embryo and 

 free sarcode globulea. Ori- 

 ginal. 



