120 



INVERTEBRATE ANIMALS. 



time in the water ; but they finally attach themselves to a blade of 

 grass, and surround themselves with a horny capsvile. If now, the 

 encapsuled embryo should beswallowed by the sheep, along with 



the grass on which it feeds, 

 the embryo escapes from its 

 liorny covering, and makes 



9 / -JA^^U \l§ ft"/ WA '\ ^^^ ^^^' ^'"°"i ^^^ stomach of 



' " the sheep into the bile-ducts, 



where it becomes developed 

 into the adult Liver-fluke. 



Order III. Turbellaria. 



The Turbellarian worms 

 differ altogether from the 

 preceding orders of Scolecids 

 in being mostly aquatic in 

 their habits and in being 

 non - parasitic. They never 

 possess sucking-discs or ceph- 

 alic hooks, and their integ- 

 ument is always furnished 

 with vibrating cilia. A 

 water-vascular system is al- 

 ways present, but it appears 

 sometimes not to communi- 

 cate with the exterior. The 

 alimentary canal is some- 

 times simply hollowed out of 

 the tissues and destitute of 

 an anus, as in the Trematoda^ 

 and 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 

 cer-tain little, soft - bodied, 

 ovate, or elliptical ci'eatures, 

 which are commonly found 

 in fresh water or oil the sea - shore, or in moist earth, and 

 are known as Planariaiis. The skin in these curious little 

 animals (fig. 75) is lichly furnished with cilia, and also contains 



Fig. 75.— One of the Turbellarian Worms (iep- 

 topUtna tremellarisX enlarged, o Mouth ; pr 

 Proboscis; g The principal nerve-ganglion, 

 jilaced in tlie anterior part of tlie body, and 

 giving off niuiierous radiating branches {n); 

 p Penis ; vd Vas deferens ; vs Vesicula seni- 

 inalis ; avi Opening of male reproductive 

 organs ; ( Testis ; or Ovary ; u Uterus, partly 

 filled with eggs ; af Opening of the female 

 repioductive oi'gans ; 7-.s Recei»taeuluni scni- 

 inis ; 17ft Alliuniiniparous gland. 



