THREAD-WORMS 447 



species just described are brightly coloured, and some of them 

 attain a large size, one recorded specimen being six inches long 

 and four broad. 



Speaking generally, the class of Turbellaria is one of extreme 

 interest, for it contains the simplest animals exhibiting bilateral 

 symmetry, and this is obviously associated with the creeping habit 

 (p. 22). 



THREAD-WORMS (NEMATHELMIA) 



This phylum comprises an enormous number of unsegmented 

 worm-like forms in which both ends of the body are usually 

 pointed. The large majority of them are parasitic in the bodies 

 of plants or animals, and various modifications in structure have 

 taken place in consequence of this habit. A common and large 

 type is the Round- Worm, one species of which (Ascaris lumbri- 

 coides) infests the human intestine, while a much larger species 

 {Ascaris megalocephald] often abounds in the corresponding part 

 of the horse. 



The body of a Round- Worm (fig. 276), though cylindrical, 

 exhibits bilateral symmetry, but not in so marked a manner as 

 in the higher worms. Jaws and paired appendages of all kinds 

 are entirely absent. The mouth is placed at the front end, and 

 is guarded by three flap-like lips, of which one is dorsal, while 

 the interspace between the other two is in the mid-ventral line. 

 Each side-lip bears a small papilla which probably has to do 

 with touch, and two of these are to be found on the dorsal lip. 

 The intestine opens on the under side, not far from the hinder 

 end, which in the male is sharply bent round. Close examination 

 shows the presence of four streaks running longitudinally along 

 the body and corresponding to special modifications of the body- 

 wall. Two of these streaks are the lateral lines, one on each 

 side, and the others are dorsal and ventral lines, running in the 

 median plane above and below respectively. 



The body-wall resembles that of a Nereis (p. 426) in so far 

 that it consists of external cuticle with underlying epidermis and 

 muscle, but the differences in detail are very considerable. The 

 cuticle is very firm and strong, probably serving to protect the 

 animal from the digestive juices of its host, while the epidermis 

 is indistinct and projects inwards to form the lateral and median 



