THE TREMATODA 69 



the intermediate host is a small crustacean, into which Bilharzia, pene- 

 trates and encysts ; the host is swallowed, with the water, by man. 

 B. magna, Cobb., in the vena cava of Cercopithecus fuliginosus. B. bovis, 

 Sons., in domestic cow ; Egypt and Sicily. Koellikeria, Cobb., unisexual, 

 dimorphic; the male thread-like, the female swollen posteriorly (Fig, 

 XI. 5) ; they live coiled together in pairs, encysted in the oral and 

 branchial cavity of marine fish. FAMILY 3. HOLOSTOMIDAE. The 

 body is divided into two regions : in the hinder, cylindrical, tail-like 

 region the genital organs are developed, the copulatory aperture of which 

 is at the posterior end, where a sucker is developed. In addition to 

 the normal two suckers, which are both situated in the anterior region of 

 the body, there is developed here an adhesive apparatus by the inrolling 

 of the sides to a greater or less extent. The members of this family 

 present no " asexual " generation in their life-history ; each egg gives rise 

 to only one sexual worm, but passes through a larval stage, which, 

 when encysted in an intermediate host, is known as " Tetracotyle," and 

 resembles a cercaria ; 1 for an account of anatomy and development, see 

 10, 12). Diplostomum, v. Nordm., in birds and crocodiles (Fig. XIII.). 

 Hemistomum, Dies., numerous species in birds and mammals (Fig. XIII. 2). 

 H. excavatum, Nit, in Ciconia alba, has a larva living in Rana temporaria. 

 Holostomum, Nit., numerous species in intestine of birds. H. variegatum, 

 in Larus ridibundus and other birds, has as larva Tetracotyle ovata, 

 v. Linstow, which occurs encysted in the peritoneum, in the head and 

 elsewhere of Acerina cornua. H. variabile, Nitzsch (Fig. XIII. 3), has as 

 larva, Tetracotyle colubri, v. Linst. Ercolani was the first to prove by 

 feeding experiments that " Tetracotyle " or " Diplostomum " develops into 

 Holostomum. Polycotyle, W. Suhm. ; P. ornata, W. S., gut of Alligator 

 lucius. FAMILY 4. MONOSTOMIDAE. The characteristic posterior sucker 

 has disappeared, but the oral sucker (everted pharynx according to Monti- 

 celli) remains. The genital pore usually occupies the normal position ; 

 there is no Laurer's canal. Monostomum, Zed. (Fig. X. 10); many species 

 in all vertebrates (see 47). Opisthotrema, Lkt. (Fig. X. 11); Notocotyle, 

 Dies., in birds (Fig. X. 5). Ogmogaster, Jaeg., in Cetacea (Fig. X. 4). 

 FAMILY 5. GASTEROSTOMIDAE. Mouth ventral ; anterior terminal sucker 

 imperforate, surrounded by tentacular-^: e processes ; genital pore at the 

 hinder end of body ; intestine sac-like, unforked, short. Gasterostomum, 

 v. Sieb., in intestine of fish (Fig. X. 8). Gasterostomum fimbriatum, 

 v. Sieb., lives in Perca fluviatilis ; the egg gives rise to a larva which 

 makes its way into Unio or Anodon; a sporocyst is formed in the liver 

 or gonad ; and within this arises the peculiar cercaria known as Bucephalus, 

 v. Baer (1), from its resemblance to the head of an ox (Fig. XIV.). 

 When liberated they make their way out through the exhalant siphon 

 and live freely in the water for a few hours. This cercaria is destined 

 to be swallowed by the second intermediate host, Leuciscus erythroph- 

 thalmus, in which it encysts itself in the wall of the mouth, or on the 

 gills, and when Leuciscus is devoured by the perch, the cyst is dissolved, 

 and sets free Gasterostomum, into which Bucephalus (as Wagener showed, 



1 In specimens of A mmocoetes great numbers of Tetracotyle sometimes occur in the 

 vascular membrane covering the brain (Brown, Qu. J. Mic. Sc. xli. p. 489, 1899). 



