CESTODA. 243 



an extent and in a manner which has not been ascertained, from the" surrounding 

 plasma. 



By some naturalists the Dicyemidae and the Orthonectidae have been regarded 

 as survivals of a most primitive Metazoan group of a group possibly inter- 

 mediate between the Protozoa and the Metazoa, and they have b^en grouped 

 together as Mesozoa. There does not, however, appear to us to be any sufficient 

 reason for this view, especially when we remember their parasitic habit. We 

 are inclined to regard them as allied to the Trematoda, to the miracidium larva 

 of which they do present some considerable resemblance. 



Trichoplax, which may be mentioned here, has been found in salt-water 

 aquaria (F. E. Schulze, Zool. Anzeiger 6). It is a small, flattened organism 

 (1-3 mm. in diameter), and consists of a sponge-work of protoplasm, with 

 nuclei at the nodes, and continuous with processes of the surface epithelial cells. 

 The latter are ciliated on the lower surface of the animal. Nothing is known of 

 the reproduction. 



Salinella is another form which may be mentioned here (Frenzel, Arch, f 

 Naturg., 58, 1891). It has only been found in aquaria. 



Order 3. CESTODA.* 



Elongated and usually segmented Platyhelminthes without mouth 

 or alimentary canal, with organs for attachment at the anterioi' 

 extremity. 



The tape-worms (Fig. 196), which may easily be recognised by 

 their band-shaped, usually segmented bodies, are parasitic in the 

 alimentary canal of Vertebrata, and were formerly taken for single 

 animals. Steenstrup was the first to introduce a different view, 

 according to which the tape-worm is a colonial animal (Strobild), 

 a chain of single animals, each segment or proglottis being an 

 individual. There are, however, Cestoda, like Caryophyllaeus (Fig. 

 212), which are destitute both of external segmentation and of 



* Besides the older works and papers of Pallas, Zeder, Bremser, Rudolphi, 

 Diesing, and others, compare van Beneden, "Les vers cestoides ou acotyles," 

 Brussels, ]850. Kiichenmeister, Ueber Cestoden im Allgemeinen und die des 

 Menschen insbesondere, Dresden, 1853. V. Siebold, Ueber die Band- und Blasen- 

 wiirmer, Leipzig, 1854. G. Wagener, "Die Entwickelung, der Cestoden," Nov. 

 Act. Leop.-Car., torn. 24, Suppl., 1854. G. Wagener, Beitrag zur Entwicke- 

 lungsgeschichte der Eingeweidewiirmer, Haarlem, 1857. R. Leuckart, Die 

 Blasenbandwurmer und ihre Entwickelung, Giessen, 1856. R. Leuckart, The 

 Parasites of Man, vol. 1, 1886, London. F. Sommer and L. Landois, "Ueber 

 den Bau der geschlechtsreifen Glieder von Bothriocephalus latus," Zeitschr. 

 f. wiss. Zool., 1872. F. Sommer, "Ueber den Bau und die Entwickelungs- 

 geschichte der Geschlechtsorgane von Taenia mediocanellata und Taenia solium," 

 Ibid., torn. 24, 1874. M. Braun, Zur Entwick. gesch. des breiten Bandivurines 

 ( Bothriocephal i 

 Entwick. 

 Nervensystei 



La Structure anat. et hist, des Cesto'ides, Geneva, 1888. B. Grassi u. G. Rovelli, 

 "Embryol. Forsch. an Cestoden," Centralbl. f. BaJcteriol., 5, 1889. M. Braun, 

 "Vermes" in Bronn's Thierreich, 4, 1895. 0. v. Linstow, Compendium der 

 Helminthologie, Hannover, 1878 ; and Nachtrag to the same, 1889. 



