i6o Introduction to Animal Morphology. 



imbedded in slime, and early develop a helmet-like- 

 larva (pilidium), with an apical flagellum, ciliated 

 sides, and a simple, inferior, alimentary canal. The 

 history of this is comparable with that of the Echino- 

 dermal pluteus, as only the digestive canal and the 

 blastema around it develop into the perfect form ; 

 the rest is only provisional. Some larvae shed only 

 their ciliated skin. 



This class contains the following orders : — 



I. Dendrocoela {Ehrenherg). — Digestive cavity tree-like,, 

 aproctous ; pharynx protrusible ; body broad and flat ; sex- 

 openings double ; mostly marine ; a few freshwater and land 

 forms. This contains two sub-orders: — i. Acephala — deve- 

 lopment often with metamorphosis ; anterior end not spe- 

 cialized into a head ; including four families : — i. Aceridse — 

 with no tentacles ; eyeless (Polycladus, Typhlolepta), or with 

 2 (Dicelis), 3 (Tricelis), 4 (Tetracelis), or many eyes (Poly- 

 celis,* Leptoplana), in which the two vasa differentia are 

 united by a common duct. 2. Pseudoceridae — with pseudo- 

 tentaculse ; the back is smooth (Eurj'lepta) or papillose 

 (Thysanozoon) ; the latter has a double penis. 3. Prosthe- 

 ceridce — tentacles true, anterior ; the pharynx is entire (Pros- 

 theceraeus, Homaloceraeus, Schmardea), or divided along it& 

 protrusible margin (Phagocata). 4. Notoceridae — tentacles 

 on back of neck; eyes none (Planacera), or at the base 

 (Stylochus), or at the point of the tentacles (Imogene). 

 Trachyplana has hyaline tentacles. 



Sub-order 2. Cephalota — prostomium differentiated ; deve- 

 lopment without metamorphosis, including two families. 

 5. Carenota — head distinct, hammer-like (Bipalium, a land 

 form from Ceylon and India), four (Cephalocarena), or three- 

 sided (Goniocarena), with two tentacles (Carenoceraeus). 

 Polycladus is eyeless. 6. Planariidae — head not very distinct 

 nor tentaculate, often with processes ; mouth medio-ventral. 

 Planaria has two eyes ; Anocelis has none ; Dendrocoelum has 



* These genera have simple development. 



