656 THE ANIMAL KINGDOM. 



by longitudinal canals, or by a network of vessels both alike furnished with 

 terminal ciliated funnels , and opening externally by one or more pores or a 

 pulsatile vesicle. Hermaphrodite. Male and female genital organs are rarely 

 present in a single set, but are usually repeated many times. Endo-parasitic. 



The sexual worm inhabits the digestive tract of some Vertebrate host, 

 with the exception of Archigetes, which becomes sexual in the coelome of 

 Tubifex rivulorum (Oligochaeta). It possesses but one set of male and female 

 genital organs, and is therefore obviously unisegmental in Archigetes, 

 Caryophyllaetis and Amphilinidae. In other instances it has many sets of 

 genitalia metamerically disposed, and the body of the worm is then usually 

 segmented to correspond. This segmentation, however, is scarcely in- 

 dicated in some instances (Triaenophorus, Ligula), and it is generally well 

 marked in the anterior region, where the rudiments of sexual organs 

 are not to be traced, and it may even appear without their development. 

 It commences anteriorly behind the head and neck or scolex (infra), and 

 the segments increase in size, &c., the further they are from this spot. The 

 terms ' joint ' or * proglottis ' are applied to the segments which are by many 

 authorities regarded as zooids produced by serial gemmation from the 

 scolex and the sexual worm or 'strobila/ therefore, as a compound or 

 colonial organism. These views are probably incorrect (see pp. 231-2), 

 and it is possible that every Cestode possesses a certain fixed limit of size 

 and growth 1 . 



The non-sexual worm rarely occurs in the digestive tract, as e. g. of 

 Lophius, but as a rule in the flesh, or one of the viscera (liver, brain, &c.) of 

 a Vertebrate. It has been found also in the coelome or tissues of some 

 Mollusca, Arthropoda, and Vermes. It is generally considered to consist 

 of two generations united together : (i) the proscolex, produced by the 

 growth of the embryo, and (2) the scolex, developed from a disc of cells in 

 the body-walls of the proscolex. The proscolex varies much in size : it is 

 usually globular but sometimes oval or band-like, and may be either solid 

 or, as in most Taeniadae, hollow. The scolex consists of a head with its 

 various organs of adhesion and a longer or shorter neck. It is attached to 

 the proscolex, and generally invaginated into it, the position in which it is 

 nearly invariably developed. The scolex may acquire a set of generative 

 organs, and remain permanently in connection with the proscolex, as in 

 Archigetes ; it may be set free and lead a wandering life, or enter an inter- 

 mediate host, as in some species of Tetrarhynchus ; and when it is trans- 



1 The scolex of Anthobothrium Musteli and Phyllobothrium Lactuca appears to increase 4-6 

 times in size after it enters its second host. The detached joints may similarly increase immensely, 

 e.g. the strobila of Calliobothrium Eschrichti is 4-5 mm. in length, the free joint 8-9 mm., and the 

 ovum, including its filaments, measures 6-7 mm. Phyllobothrium thridax, Anthobothrium cornu- 

 copia, and Tetrarhynchus minutus may also be instanced, but the contrast between the strobila and 

 the joint is not so extreme. 



