438 ZOOLOGY. 



alimentary canal, or endoderm ; consisting in the adult of 

 a rounded head bearing suckers and hooks for adhesion, 

 and budding off a series of segments called proglottides, 

 each containing a complete set of hermaphrodite repro- 

 ductive organs. The life-history consists of two stages 

 passed in separate hosts, usually both Vertebrate; the 

 larva is sexually immature, rarely reproduces itself 

 asexually, and develops directly into the sexually mature 

 adult. 



The proglottides are to be regarded as sexual indi- 

 viduals produced by gemmation from an asexual individual, 

 so that the Cestoda or Tape-worms present the pheno- 

 menon of metagenesis. 



The larva is known as a Gysticercus or Cysticercoid, 

 consisting of a more or less enlarged hollow vesicle in- 

 vaginated at one point ; hooks and suckers are developed 

 on the invaginated surface, which is afterwards everted to 

 form the head of the adult. When the larva is swallowed 

 by the final host the head of the tape-worm becomes 

 attached to the wall of the intestine and the proglottides 

 are budded from the head in succession, so that the 

 youngest are always next to the head. As the proglottides 

 get older the generative organs begin to develop in them, 

 the male parts first. In each proglottis is formed a com- 

 plete set of these organs, which consists of (i) a large 

 number of small rounded testes, whose ducts unite and 

 lead finally to a single vas deferens opening at the lateral 

 margin of the proglottis, and (ii) a pair of ovaries placed 

 near the posterior border of the proglottis, and opening 

 into the oviduct, whose external aperture is at the side of 

 the male aperture. Behind the ovaries is situated a yolk 

 gland, whose duct opens into the oviduct, and a shell 

 gland opens into the yolk duct. Leading from the oviduct 

 is a uterus for the storage of fully formed eggs, and 

 beyond this is a receptaculum seminis. The end of the vas 

 deferens forms a protrusible intromittent organ or cirrus, 

 but it is not known whether copulation occurs between 

 cirrus and vagina of the same or of neighbouring seg- 

 ments. In the oldest segments large numbers of fertilised 

 eggs accumulate in the uterus, while the rest of the 



