TTJEBELLAEIA. 9 



an independent existence. Sir John Graham Dalyell saw one 

 individual {Planaria nigra) voluntarily decapitate itself, and by 

 artificial experiment he has also shown that " innumerable sections 

 of the body" in this species are all capable of becoming " complete 

 and perfect animals." Other observers, such as Duges and J. R. 

 Johnson, have obtained similar results. As regards their develop- 

 ment from the egg much remains to be done, but Agassiz states 

 that the infusorial genera Kolpoda and Paramecium are only 

 embryos of Planaria. According to Yon Siebold, the eggs are 

 multiple (as occurs in certain tapeworms — Toinia cucumsrina for 

 example), and develop several embryos simultaneously. These 

 embryos subsequently become clothed with ciha, and without any 

 alternation of generation ultimately acquire the form and structure 

 of their adult progenitors. 



As above indicated, all the true Planarians (Dendrocoela) display 

 a branched digestive tube, but a second group of the family {Bhah- 

 docoela) have a simple intestine, combined with a closer union of 

 the male and female reproductive organs, except in one genus 

 {DinojpJiilus) where the sexes are altogether separate. In other 

 words, the sexual systems are so united that they present only a 

 single outlet, as may be seen, for example, in the well-known genus 

 Mesostoma, which may be taken as a type of the rhabdocoelian 

 group. The different species of this genus vary from three or four 

 hues to about three quarters of an inch in length, and, according to 

 the observations of Focke, they are capable of throwing themselves 

 into an endless variety of forms, occasionally folding their lateral 

 borders in such a manner that the body becomes doubled, as it 

 were, somewhat after the fashion of the Siamese twins.* 



Genera. — Dendrocoelian group, — Planaria, 0. F. Mueller (in 

 part), Ehrenberg ; Monocelis, Ehrenberg, Oersted ; Polycelis, 

 Ehrenberg ; = Prosthiostomum, Quatrefages ; Tricelis, Ehrenberg ; 

 Planocera, Mertens ; Leptoplana, Ehrenberg ; Eurylepta, Mortens ; 



* On Planaria Ehrenbergii, in the " Ann. des Wiener Museums,^^ for 1836, vol. i., p. 195. 

 See also Oskar Schmidt's memoir " Die Rliahdocoelen Striidehcurmer,^^ in the Vienna 

 Academy's Transactions, 1858. 



C 



