242 rLATIlELMIXTHES 



to water plants. !Man}' species form a sort of cocoon, containing a few 

 eggs and numerous yolk cells, There are a few viviparous forms (summer 

 generation of ^Mesostomida-). In lire marine species a free-swimming 

 larva (protrocliula, lig. Jio), with lol)e-like processes may hatch from tlie 

 egg. This larva, bv a metamorphosis, is converted into the creeping adult. 

 Not infrec[ueutlv besides the sexual, ase.xual reproduction occurs. The 

 ]Microstomid:e and some Plaitariw are capable of transwrse di\ision, and 

 will form chains of individuals. For each posterior indi\'idual a new 

 brain and a new a^sophagus are formed (llg. 50). The Turbellaria 

 possess a marked power of reproducing lost parts, making them favorites 

 for regeneration experiments. 



Tn a few Turbellaria the pharynx connects with a solid protoplasmic mass, 

 in which, as in the protoplasm of a protozoan, the food is tligested. This ento- 

 derm is hardly marked off from the mesoderm, but it is a question whether these 

 'Aca:la' are priniiuve or degenerate, the latter being the more probable. 



Order I. Polycladidea. 



Marine species of consideralile size, digestive ea'ca springing from a central 

 chamber. Lcploplaiid,''' SlyUhiiits:* TJiysanozoon, Europe. 



Order II. Tricladidea. 



.\limentary canal widi an anterinr unpaired anil a pair of posterior branches, 

 arising from the pharynx, and hearing lateral ea-ca. Marine, BdcUoiira* and 

 SyncaUdiunfi (fig. 215) (parasitic on Liniiiliis). Chz/i/i;,* ]\ilYiiiirriis:* fresh- 

 rcater, Di-iidroi-a-liiiii"'' (fig. 64), Phiitaridr' and Polvxcrlisr- Phdi;ocata* with 

 divided pharynx. The tropical land planarians (Jiipaliinii* 10 or 12 inches 

 long) have been introduced into greenhouses. 



Order HI. Rhabdocoelida. 



Small, even microscopic, recalling in habits and ap]iearance (he Infusoria; 

 alimentary canal rod-like, without liranches. ]'(iiirx''' (fig. 75), fresh water; 

 ^[ol!ops* M on osrcli. <:,'■' marine. The fresh-water Microstcuud.e reproduce 

 almost exclusively by fission. 



Class II. Trematoda. 



These are exclusiyely parasitic, some living on the skin or gills (ecto- 

 parasites) or in the interior of other animals (entoparasites). In structure 

 they are closest to the trichnl Turbellaria, from which they differ by 

 characters, the direct result of their parasitic life. Thus they have lost 

 the cilia or have them only in the larva. On the other hand, tltey are 

 covered with a cuticle often with spines and with suckers and hooks for 

 adhesion to the host. The suckers are shallow pits of columnar epithelium 

 lined with cuticle and furnished with a thick layer of radial and circular 

 muscles, which by their contraction increase the lumen of the sucker, the 

 edges of which are closely applied to the host. At least one such sucker 



