_Distomum macrostomum._—This form occurs in the intestine of several common Passerine birds. It is remarkable not only for the large oral sucker, but also on account of the position of the common genital pore at the hinder, and not as usual, at the anterior, end of the body (Fig. 32, A). The eggs pass out through this pore, and are discharged with the bird's excrement. Should a certain snail (_Succinea putris_) happen to rasp off the epidermis of a leaf upon which the faeces have fallen, the eggs are swallowed and a minute active larva is set free (Fig. 32, B). This penetrates through the thin wall of the digestive tract of the snail, and passing into the connective tissue, throws off its cilia and assumes the shape of Fig. 32, C. This _sporocyst_, as the larva is now termed, grows rapidly in all directions (Fig. 32, D) at the expense of the snail's tissues, until it becomes impossible to separate parasite and host completely. {65}[Illustration: FIG. 32.—Life-history of _Distomum macrostomum_ Rud. A, Immature _Distomum_ (really a tailless _Cercaria_) found in the swollen terminal parts of _Leucochloridium_ (Fig. 33, B) and enclosed in two protective membranes, × 40; B, larva which hatches out of the egg of _D. macrostomum_, × 125; C, the metamorphosed larva (sporocyst) fourteen days after having entered _Succinea putris_, and pierced through its intestinal wall; D, actively growing sporocyst. (After Heckert.) _go_, Genital aperture; _int_, intestine; _ms_, mouth sucker; _n_, nervous system; _ov_, ovary; _ps_, ventral sucker; _te_, testis.] Those branches which 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 condition 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-branch. A sphincter muscle closes the cut end of the fertile sac when the bird's bill nips it off. The sac contains large numbers 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 {66}nestlings by the unintentional agency of their parents.[79] The snail regenerates its lost tentacles only for the sporocyst to again bud off fertile branches into them.