SCOLECIDA. 



113 



OKDER HI. TURBELLARIA. The animals included in this 

 order differ altogether from the Trematoda and Tceniada in 

 being- almost all aquatic in their habits and being all non- 

 parasitic. They never possess sucking-disks or booklets, and 

 their integument is always furnished with vibrating cilia. A 

 water-vascular system is always present, but it appears some- 

 times not to communicate with the exterior. The alimentary 

 canal is sometimes simply hollowed out of the tissues and 

 destitute of an anus, as in the Trematoda, or at other times 

 suspended in a free space (body-cavity) and furnished with an 

 anus. It may be simple or much branched. 



The best known of the members of this order are certain 

 little jelly-like, soft-bodied, ovate, or elliptical creatures, which 

 are commonly found in fresh water or on the sea-shore, and are 

 known as Planarians. The skin in these curious little ani- 

 mals (Fig. 41, 1, 2) is richly furnished with cilia, and also 

 contains numerous cells which have been compared to the 



FIG. 41. Turbellaria. 1. Planaria torva: m Mouth; (7 Nerve-ganglion ; e Eyes; ov 

 Ovary; t Spermarium ; gn Genital opening: 2. Planaria, lactea, showing the branched 

 intestine: 3. Larva of one of the marine Turbellarians : 4. Pilidium, the larva of one 

 of the Nemertidm. 



"nettle-cells" of the Ccelenterata. The intestine may be 

 either straight or branched, but always terminates behind 

 in blind pouches, and is never provided with an anus. The 

 water-vascular system communicates with the exterior. The 

 nervous system consists of two ganglia, placed in front of the 

 mouth, and united by a cord. There are generally rudimen- 

 tary eyes or pigment-spots, which vary in number from two 

 to sixteen. 



The remaining members of the Turbellaria are known as 

 ribbon-worms (Nemertidve), and are not uncommonly found 

 on the sea-shore. They differ from the Planarida in beinpf 



