MALACOTYLEA. 237 



Fani. Aspidobothridae. With the characters of the order. Aspidogaster 

 v. Baer ; A. concliicola v. Baer, from the pericardial cavity of Anodonta and Unio. 

 Plutyaspis Mont.; Cotylogastcr Mcatic ; Macraspis Olss. 



Order 3. MALACOTYLEA = DIGENEA. 



Body variously shaped. Being internal parasites the adhesive 

 apparatus is, as a rule, feebly developed, consisting of two suckers, 

 one the oral sucker at the anterior end, and the other (less 

 frequently present) on the ventral surface, or at the hind end. An 

 armature of chitinous hooks is never present on the posterior sucker, 

 and only exceptionally on the anterior sucker. Additional adhesive 

 organs are occasionally present (Holostmnatidae, Amghistomatidae). 

 Mouth terminal or sub-terminal, exceptionally on the middle of the 

 ventral surface (Gasterostoinum), almost always surrounded by a 

 sucker ; eyes exceptionally present. Excretory organs open posteriorly 

 by a terminal or dorsal pore. Hermaphrodite, except Billiarzia and 

 possibly one or two other forms.* Genital organs open close to one 

 another, or into an atrium. Genital pores usually on the ventral 

 surface anteriorly, rarely placed posteriorly, or laterally, or at the 

 front end. One ovary and usually two testes ; yolk-gland paired, 

 rarely unpaired. Laurer's canal usually present. Eggs usually 

 without filaments, with operculum. Life-history with alternation 

 of generations (digenetic, except Holostoinidae) and more than one 

 host. The sexual form is parasitic in the alimentary canal, and its 

 appendages, of Vertebrates f ; the asexual generations in Mollusca, 

 and the encapsuled larva in Invertebrata and lower Vertebrata, 

 Insectivorous birds are particularly infested by them. It may be 

 of interest to youniiets to know that the trail of a woodcock 

 largely consists of distomic Trematodes. 



Fani. 1. Holostomatidae (Metastatica). With two suckers, and a peculiar 

 adhesive apparatus behind the ventral sucker on the anterior region of the body. 

 Body divided into an anterior flattened and a posterior cylindrical region (Fig. 

 189). Genital organs in the posterior region, and genital opening at the hind 

 end. In the alimentary canal of birds, reptiles, and mammals, rarely in am- 

 phibia and fishes. The eggs are large and not very numerous, and hatch out 



* Dimorphic forms are found in certain species of Monostomum and Distomum 

 in connection with the division of labour of the sexual functions ; one individual 

 develops only male organs, and the other only female, the former producing 

 spermatozoa, and the latter ova. The vestige of the functionless generative 

 gland undergoes in these cases a more or less complete degeneration. Such forms 

 are morphologically hermaphrodite, but practically of separate sexes. 



t There are a lew exceptions to this rule, e.g., Distomum Echiuri in the 

 nephridia of Echiurua pallasii, and D. rhizophysac in the gastrovascular 

 apparatus of the siphonophore Ilhizophysa conifera. 



