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MANUAL OF ZOOLOGY. 



Other members of the order infest the intestines of birds and 

 Batrachians, the gills of fishes, or the paunch of Ruminants. 



ORDER TURBELLARIA. The members of this order are al- 

 most 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 cili- 

 ated, and they never possess either suctorial discs or a circlet 

 of cephalic hooklets. A " water-vascular system " is always 

 present, opening externally by one or more apertures, or ap- 

 pearing to be entirely closed in the adult (Nemertida). As in 

 the Trematoda, the alimentary canal is imbedded in the paren- 

 chyma of the body, and, except in the Nemertida, there is no 

 " perivisceral cavity." The intestine is either straight or 

 branched, and a distinct anal aperture may, or may not, be 

 present. The nervous system consists of ganglia situated 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 re- 

 spectively the Planarida and the Nemertida. 



SUB-ORDER I. PLANARIDA. The Planarians (fig. 48) are 

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

 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 



Fig. 48. Morphology of Turbellaria. i. Planaria torva (Miiller) ; in 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 laur- 

 ina, a marine Turbellarian. 4. Pilidium, the " pseudembryo " of a Nemertid ; a 

 The alimentary canal ; b Rudiment of the Nemertid. 



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, constituting 



