LIFE-HISTORIES OF DISTOMATIDAE 



lie superficially in the cephalic region of the snail become greatly 

 swollen, cylindrical, and contractile. They are banded with green 

 and white, ornamented with red terminal spots, and pulsate rapidly. 

 Hence these fertile branches of the sporocyst (which in this con- 

 dition was known as Leucochloridium paradoxum, Fig. 33, B) 

 naturally attract the attention of insectivorous birds, which peck 

 off the tentacles of the snail, and with it the swollen sporocyst- 

 brauch. A sphincter muscle closes the cut end of the fertile sac 

 when the bird's bill nips it off. The sac contains large numbers 



FIG. 32. Life-history of Distomum 

 macrostomum Bud. A, Immature 

 Distomum (really a tailless Cer- 

 caria) found in the swollen ter- 

 minal parts of Leucochloridium 

 (Fig. 33, B) and enclosed in two 

 protective membranes, x 40 ; B, 

 larva which hatches out of the 

 egg of D. macrostomum, x 125 ; 

 C, the metamorphosed larva 

 (sporocyst) fourteen days after 

 having entered Succinea putris, 

 and pierced through its intes- 

 tinal wall ; D, actively growing 

 sporocyst. (After Heckert. ) go, 

 Genital aperture ; int, intestine ; 

 ms, mouth sucker ; n, nervous 

 system ; oi>, ovary ; ps, ventral 

 sucker ; te, testis. 



Xr n. 



pa- 



of young D. macrostomum (Fig. 32, A), produced by the division of 

 embryonic cells of the larva (Fig. 32, B), which are apparently 

 blastomeres of the egg reserved for this future use. It is a 

 remarkable circumstance that the old bird itself is immune from 

 infection, and if it swallows these young Distomes, they are 

 digested. Should, however, the snail's tentacle and its contents 

 be offered as food to the nestlings, their weaker digestive powers 

 merely set the Distomes free from the protective membranes (Fig. 

 32, A), and thus the Blackcaps, Sparrows, and other birds infested 

 by D. macrostomum have acquired the parasite when they were 

 VOL. n F 



