58 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.55. 



dian in position and about halfway between the plane of the 

 genital pore and the posterior border of the segment. The 

 vitellarium is situated near the posterior border of the segment; it 

 consists of two lobes, a dorsal and a ventral, lying one above the 

 other, the lobes in turn being formed of two portions, each with its 

 own duct. The shell-gland is spherical to oval. The ovaries, vitel- 

 larium, and uterus have the common openings of their tubes within 

 the shell-gland. The vagina ha3 a large, elongate, setose dilation 

 about 60 \x in diameter near the genital pore and bends at an obtuse 

 angle median of this to extend posteriorly and medially to a recep- 

 taculum seminis about 14 \i in diameter. The uterus develops com- 

 paratively late, forming a median stem and lateral enlargements, the 

 outlines of these later becoming quite indistinct. The eggs are 32 to 

 36 PL by 25 to 30 [i, in diameter. 



Hosts. — Primary: Canis familiaris, C. lupus, C. aureus, C. dingo, 

 C. mesomelas, Felis catus {F. c. domestica), F. concolor. Second- 

 ary : Homo sapiens, Bos taurus, Ovis aries, Capra hircus, Sus scrofa 

 domestica, S. scrofa, Pithecus species (Simia cynomolgus) , Pithecus 

 silenus {Simia silenus), Simia sylvanus {Simia inuus), Ovis ammon, 

 Camelus hactrianus, C. dromedarius, Giraffa species {C amelopardalis 

 giraffa), Tetraceros quadricornis, Alces alces, Equus cdbaUus, E. 

 zehra, E. asinus, Tapirus indicus, Canis familiaris, Felis catus {F. c. 

 domestica), F. pardus, Macropus ma'pr, Sciurus vulgaris, Mungos 

 ichneumon {Herpestes ichneumon), Simia sylvanus {Inuus ecau- 

 datus), Macropus giganteus, Ovis argali, Tapirus ameHcanus, Oryc- 

 tolagus cuniculus {Lepus cuniculus). 



Location. — In small intestine of primary host. In practically 

 every organ and tissue of the secondary hosts. 



Localities. — Cosmopolitan. 



Life history. — The eggs produced by the adult worm in the intes- 

 tine of the primary host pass out and are ingested by the secondary 

 host in contaminated food or water, as a rule. In the digestive tract 

 the embryo escapes from the shell and makes its way into the tissues. 

 Here it develops into the larval stage, a bladderworm which may 

 attain the size of a child's head, commonly called an echinococcus 

 or hydatid, and known under numerous scientific names, largely based 

 on morphological variations or hosts, and characterized by the forma- 

 tion of a laminated external layer and a delicate internal germinal 

 layer. As a defense reaction against unfavorable conditions, the 

 internal genninal membrane or scolices derived from it may give 

 rise to internal daughter bladders, while proliferative elements in 

 the cyst wall may form external daughter bladders. Daughter 

 bladders may form grand-daughter bladders and any bladder of the 

 series may form brood capsules containing scolices. When the larvae 

 are ingested by the primary host, the heads contained in these brood 

 capsules develop segments and form the strobilate worm. 



