annuloida: scolecida. 173 



In Distoma lanceolatum (fig. 49, 2) the intestine has not the 

 ramose, complex character of that of D. hepaticum. On the 

 other hand, the alimentary canal, after its bifurcation, is con- 

 tinued on each side of the body to the posterior extremity 

 without giving off any branches on the way, and it terminates 

 simply in blind extremities. 



JDtplostoinuin, in its essential characters, does not differ 

 much from Distoma; but it is found living gregariously in the 

 vitreous humour and lens of the eyes of certain fresh-water 

 fishes, such as the common Perch. 



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 

 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 cili- 

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

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

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

 jjearing to be entirely closed in the adult {Nemertidd). 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 Plaharida and the Nemertida. 



Sub-order I. Planarida. — The Planarians (fig. 50) are 

 mostly ovoid or eUiptical 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 

 vibratile cilia, which subserve locomotion, and it also contains 

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

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

 portion of the body situated in front of the mouth, constituting 

 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 often evert- 

 ible ; and the intestine may be either straight or branched, 

 but always terminates caecally behind, and is never provided 

 with an aiial aperture. The " water-vascular system " commu- 



