ANNULOSA : TURBELLARIA. 



239 



The members of this order are almost all aquatic, and are all 

 non-parasitic; thus differing entirely from the animals which 

 compose the two preceding orders. Their external surface is 

 always and permanently ciliated, and they never possess either 

 suctorial discs or a circlet of cephalic hooklets. A "water- 

 vascular system" is present, opening externally by one or 

 more apertures, or appearing to be entirely closed in the adult 

 (Nemertida]. The alimentary canal is embedded in the paren- 

 chyma of the body (Planarida), or is freely suspended in a 

 " perivisceral cavity" (Nemertida). The intestine is either 

 straight or branched, and a distinct anal aperture may, or may 

 not, be present. The nervous system consists of ganglia situ- 

 ated in the fore-part of the body, united to one another by 

 transverse cords, and sending filaments backwards. 



The Turbellaria are divided into two sections, termed 

 respectively the Planarida and the Nemertida. 



SUB-ORDER I. PLANARIDA. The Planarians (fig. 117) are 

 mostly ovoid or elliptical in shape, flattened and soft-bodied. 



Fig. 117. Morphology of Turbellaria. i. Planaria torva (Miiller) : m Mouth; g 

 Nerve-ganglion; e Eyes; ov Ovary; t Testis; gn Genital opening. 2. Planaria 

 lactea, showing the branched (dendrocoel) intestine. 3. Microscopic larva of A lau- 

 rina, a marine Turbellarian. 4. Pilidium, the " pseudembryo " of a Nemertid : 

 a. The alimentary canal ; b Rudiment of the Nemertid. 



They are for the most part aquatic in their habits, occurring 

 in fresh water, or on the sea-shore, but occasionally found in 

 moist earth. The integument is abundantly provided with 

 vibratile cilia, which subserve locomotion, and it also contains 

 numerous cells, which have been compared to the " cnidae," or 

 nettle-cells, of the Ccdenterata. There is always a considerable 

 portion of the body situated in front of the mouth, constitut- 

 ing the so-called "prse-oral region," or " prostomium ; " and 

 this is often modified into a singular protrusible and retractile 

 organ, called the " proboscis," the exact use of which is not 

 known. The mouth opens into a muscular pharynx, which is 



