72 THE TREMATODA 



The early stages of development up to the formation of a free- 

 swimming larva are best known for Dist. terelicolle, from the pike 

 (Schauinsland, 39). The egg cell is embedded near the anterior 

 end of the egg in a mass of yolk cells ; segmentation is total, and 

 nearly regular, giving rise to a solid blastosphere (Fig. XV.). 

 At the anterior pole one cell delays in its further segmentation, 

 whilst the rest continue to divide and give rise to smaller cells ; 

 this apical anterior cell flattens out and divides into two ; these 

 (after further subdivision) spread over the yolk cells, which have 

 in ' the meantime diminished in size and undergone a certain 

 amount of disintegration. Other flat cells make their appearance 

 posteriorly, and extend forwards ; in this way a " yolk envelope " 

 of flat cells is formed which lies immediately within the egg 

 shell, and is left behind within it when the larva escapes. This 

 envelopment of yolk by blastomeres is similar to what happens 

 in Triclads. Meanwhile, the other blastomeres have become 

 differentiated into a flat epiblast, which becomes ciliated, and a 

 central mass of cells in which, later, the enteron becomes marked 

 out (? by delamination) as a simple sac. 



Between the enteron and the epiblast there is developed from 

 the central mass two layers of muscles, which are in many cases 

 separated from the gut by a distinct cavity, which is lined by a 

 layer of cells and may be regarded as coelom. The coelom may, 

 however, in other cases be more or less blocked up by cells of an 

 embryonic character, probably derived directly from blastomeres. 



The egg, having passed out of the host's body with the faeces, 

 undergoes its development in the water. The young larva, or 

 " miracidium " (M. Braun), or " ciliated embryo " (auctorum), now 

 leaves the egg, and the further history has been most fully studied 

 in the case of D. hepaticum (by Leuckart, 27 ; and Thomas, 43), 

 and in D. cygnoides (by Wagener, 45). The eggs of these flukes 

 pass out of the host with the faeces, as King was the first to show 

 for the liver-fluke (1836); and the miracidium escapes into the 

 water. It is to all intents and purposes a Rhabdocoelous Tur- 

 bellarian without gonads ; its shape and structure are shown in 

 Fig. XV. 6, 7, and Fig. XVI. 1. 



At the apex of the snout, which is moved by muscles, is 

 situated the mouth armed with a stylet in the case of D. 

 lanceolatum and others which leads into the short, sac-like enteron. 

 Locomotion is effected by the ciliated epiblast, aided by the 

 somatic muscles. Cilia are generally regularly developed all 

 over the surface, or in some species limited to definite areas, but 

 are never in bands ; a pair of eyes recalling in their structure 

 those of adult Heterocotyleans rests upon the brain ; a pair of 

 flame cells represents the excretory system, and are said to be 

 derived from the epiblast (Fig. XV. 6, m). 



