TURBELLABIA 157 



called Platelmia, while of the three orders of roundworms he formed the 

 class Nematelmia, an arrangement which is still maintained. 



The subphylum contains 4 classes. 



Key to the classes of Plathelminthes : 



Ci No anus ; no blood vessels ; animals mostly hermaphroditic, with very 

 complex genital organs. 



61 Animals with rare exceptions free-living ; body ciliated externally. 



1. TURBELLARIA 



62 Animals parasitic ; not ciliated externally ; mouth when present at for- 



ward end (with some rare exceptions). 

 Ci Intestine and mouth present ; animals small and unsegmented. 



2. Trematodes 

 Cj Intestine and mouth absent ; animals usually long and segmented. 



3. Cestodes 

 Oa Anus, anterior proboscis, and blood vessels present ; animals mostly 



unisexual and free-living, usually long and bandlike 4. Nemertea 



Class 1. TURBELLARIA.* 



Soft, free-living flatworms, mostly mider an inch in length, which are 

 found either in the water creeping slowly over stones or plants or living 

 in moist places on the land. The body is flat in shape and usually elon- 

 gate, but in some cases nearly circular. The external surface is ciliated 

 and from it is exuded the slimy secretion of numerous glands, in which 

 are often contained minute rod-like bodies called rhabdites which are pro- 

 duced in certain glandular cells either of the integument or of the paren- 

 chyma. A few turbellarians possess functional nettle cells which, however, 

 they have acquired from hydrozoans they have eaten, and a few have 

 adhesive papillae or suckers. 



No body cavity is present, the spaces between the organs being filled 

 with the parenchyma. The mouth (Fig. 263) is usually near the middle of 

 the ventral surface but may in the different species vary in position from 

 the forward to the hinder end. It opens into a muscular pharynx which 

 is usually of large size and one of the most prominent organs in the bodj' : 

 it can usually be thinist out of the moiith so as to form a proboscis by 

 means of which the animal takes and often digests its food. An intestine 

 is not present in the Accela : in the other turbellarians it is either a tubular 

 or a branched structure. An anus is not present, fecal matter being dis- 

 charged through the mouth : in certain cases, however, the intestinal 



* See "Rep. Invert. Vine. Sd.," by A. E. Verrill, Rep. U. S. Com. Fish, for 1871 

 and 1872. "Beob. iiber die Susswasser Turbel. Nordani.," by W. A. Silliman, Zeit. 

 f. wiss. Zool., Vol. 41, p. 48, 1885. "Turbellaria," by L. von Graff, Bronn's Kl. u. 

 Ord., Vol. 4, Abt. 1, Acoela und Rhabdocoolida, 1904-08. "Turbellaria," by same. 

 "Die Siisswasserfauna Deutscblands," 1909. "Vergleichung der Nordamerlkanischcn 

 und Europaischen Turbellarienfauna," by same, Proc. Sev. Int. Zool. Cong., 1910. 

 "Acoela, Rhabdocoela, und Allceoccela des Ostcns der Vereinigten Staaten," &c., by 

 same, Zeit. f. wiss. Zool., Vol. 99, p. 321, 1911. 



