242 PLATHELMIXTHES 



to water plants. Many species form a sort of cocoon, containing a few 

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

 generation of Mesostomicke). In the marine species a free-swimming 

 larva (protrochula, fig. 216), with lobe-like processes may hatch from the 

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

 Not infrequently besides the sexual, asexual reproduction occurs. The 

 Microstomidae and some Planar ice are capable of transverse division, and 

 will form chains of individuals. For each posterior individual a new 

 brain and a new oesophagus are formed (fig. 59). The Turbellaria 

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

 for regeneration experiments. 



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

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

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

 'Accela' are primitive or degenerate, the latter being the more probable. 



Order I. Polycladidea. 



Marine species of considerable size, digestive caeca, springing from a central 

 chamber. Leptopland* Stylochus;* Thysanozoon, Europe. 



Order II. Tricladidea. 



Alimentary canal with an anterior unpaired and a pair of posterior branches, 

 arising from the pharynx, and bearing lateral caeca. Marine, Bdelloura* and 

 Synccelidium* (fig. 215) (parasitic on Limulus), Gunda,* Polychcerus;* fresh- 

 water, Dendroca>lum* (fig. 64), Planaria,* and Polyscelis;* 'Phagocata* with 

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

 long) have been introduced into greenhouses. 



Order III. Rhabdoccelida. 



Small, even microscopic, recalling in habits and appearance the Infusoria; 

 alimentary canal rod-like, without branches. Vortex* (fig. 75), fresh water; 

 Mono ps* Monoscelis* marine. The fresh- water MiCROSTonnxE reproduce 

 almost exclusively by fission. 



Class II. Trematoda. 



These are exclusively 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 triclad 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, they 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 



