82 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



"seems to indicate that the miracidum enters the 

 shell aperture and makes its way up between the 

 shell and the body of the enclosed animal. The 

 miracidium probably penetrates the tunica propria 

 of the digestive gland . . . and then liberates the 

 contained redia by decomposition of itself." This 

 sequence of events cannot be observed in the in- 

 tact animal. The following observations demon- 

 strate that it is not the sole means of entry of 

 the parasite into the snail. 



The main attack sites are the outer wall of the 

 siphon, the base of the head, and the side (rarely, 

 the sole) of the foot. Occasionally, miracidia, 

 trapped in the siphonal water current, are swept 

 into the mantle cavity. Most, if not all, pass 

 out in the excurrent flow, but a few may possibly 

 invade the host tissues there. 



The invasion, most readily studied in the outer 

 wall of the extended siphon, occurs in the follow- 

 ing manner. The miracidimn contacts and ad- 

 heres to the skin of the drill by its anterior end. 

 Attachment and penetration occur only when the 

 miracidium strikes more or less at right angles 

 to the skin surface; miracidia striking at acute 

 angles ricochet and do not become attached. 



Immediately on attachment several longitudinal 

 contractions of the anterior two-thirds of the para- 

 site's body follow, so that it appears to butt the 

 snail. It quivers very rapidy for a few seconds, 

 becomes quiescent for a short time, then a series 

 of rhythmic contraction waves sweeps over the 

 miracidium for approximately 30 minutes before 

 slowing markedly. The contraction waves appear 

 to aid penetration (fig. 1, a to e). 



Figure 1. — Parorchis miracidia invading siplion of Thais 

 (diagrammatic). 



During the first 15 minutes (approximately) of 

 the invasion, occasional small showers of host 

 epithelial cells are carried away from the invasion 

 site by water currents produced by the parasite's 

 cilia (fig. 1, e). This observation appears to cor- 

 roborate Rees' (1940) statement that the droplets 

 of secretion seen by her to emerge from the two 

 anteriorly located penetration glands onto the 

 rostrum of the miracidium probably facilitate 

 enti-y of the miracidium into the molluscan host. 



About an hour after contact, the parasite's cilia 

 appear to stop beating. Closer examination re- 

 veals that the lateral cilia continue to beat very 

 slowly for at least another half hour as the 

 miracidium moves in and out of the low wheal 

 which develops at the invasion site and progres- 

 sively enlarges as the miracidium penetrates 

 deeper into the snail (fig. 1, g to i). 



In about II/2 hours after attachment, the first 

 generation redia contained in the miracidium 

 could no longer be seen. Wliether the redia has 

 left the miracidium and entered the host tissues or 

 still lies within the miracidimn could not be 

 ascertained. 



Complete penetration by the miracidium re- 

 quires about 6 hour's. The subsequent fate of the 

 miracidium is unknown, but it is presumed to 

 disintegrate completely within the liost, since no 

 trace has been found in serial sections of snails 

 fixed as early as 24 hours after penetration was 

 known to have occurred. Nor was Rees (1940) 

 able to find any trace of it either inside or outside 

 the snail host. On the other hand, the miracidium 

 of P. acantJitis may behave in the manner described 

 by Stunkard (1934) for Typhlocoelum eymbium. 

 whose miracidium does not penetrate the snail. 

 After that miracidium is securely attached and 

 partially embedded, the redia leaves it and enters 

 the snail. 



Peculiarly, as the miracidium penetrates deeper, 

 its eyespot seems to move posteriorly initil it lies 

 in the rearmost part, of the parasite's body (fig. 1, 

 e to i). The reason for the apparent rearward 

 migration of the eyespot is uncertain, but it is 

 probably related in some way to the escape of 

 the redia from the miracidium. 



Redml Development. — The young, first-genera- 

 tion redia has not yet been demonstrated at the 



