440 FRESH-WATER BIOLOGY 



with shell gland between. Eggs thin-shelled, no lid; onchosphere with one or more mem- 

 branes. Bladder-worm in vertebrates and invertebrates. 



The great majority of forms commonly designated Taenia are included here. Number 

 and form of hooks on which older systems are based iorm unreliable means for the distinction 

 of species. In immature forms the organs utilized in this key are undeveloped and a deter- 

 mination can only be approximate. 



55 (117) Body flattened. Distinct and regular external boundaries corre- 



sponding to internal grouping of organs in the strobila. 56 



56 (57) Suckers carry on anterior and lateral surface auricular appendages. 

 Vitellarium anterior to ovary. 



Family Tetrabothkudae Fuhrmann 1907. 



Scolex unarmed, without rostellum. Neck short. Proglottids 

 except oldest always much broader than long. Reproductive organs 

 single in each proglottid. Genital pores unilateral; genital cloaca 

 deep. Cirrus-pouch small and nearly spherical, united with genital 

 cloaca by muscular cloacal canal. Eggs with three transparent en- 

 velopes. Adults in birds and mammals. 



Tjrpe genus. Tetrahothrius Rudolphi 1819. 



With characters of the family. 



Scolex unarmed, quadrate. Suckers large. Sexual pore always 

 dextral. 



The hosts are aquatic birds, largely marine. Nearly twenty species 

 Fig. 744. Tetrahothrius j^j-g described, a number of which occur in North American birds: 

 Sfied (After LUheO ™^^" (S""' S'^^' h"0°' 1°°°)' ^^""^ frequent fresh-water bodies. 



57 (s6) Suckers simple without appendages of any sort. ViteUaria not 



anterior to ovary but posterior to it, or in the same trans- 

 verse plane with it. 58 



58 (59) Genital pores median, on flat surface of proglottids. 



Family Mesocestoididae Fuhrmann 1907. 



Scolex without rostellum or hooks. Suckers unarmed. Reproductive organs single in 

 each proglottid. Genital pores median on ventral surface. Vagina opens in front of or beside 

 cirrus-pouch. Eggs in terminal proglottids inclosed in single thick-walled egg-capsule. Adults 

 in mammals and birds. 



Type genus Mesocestoides ^'aiLlant 1863. 



With characters of the family. 



Few species; almost never in aquatic forms. No North American records although the 

 genus occurs here. 



59 (58) Male genital pores at margin or very close to it. Female pores 



when present similarly located. . 60 



60 (104) Female genital pore present and located near male pore. . 6i 



61 (103) Uterus transverse or irregular, not elongated in median line of 



proglottid. . . ... . ... 62 



62 (102) Reproductive organs simple in each proglottid or if genital pores 



are double, the organs are also double 63 



63 (64) Scolex provided with three to many rows of hooks. 



Family DrPYLiDirDAE Liihe 1910. 



No forms in fresh-water hosts. 



64 (63) Scolex provided with one or two rows of hooks or without any 



hooks 65 



