434 



CHARACTERS OF INVERTEBRATE ANIMALS 



times. When the body is fully extended the mouth will be seen 

 at its tip surrounded by a series of short, grooved tentacles. This 

 creature departs further than Echiurus from the typical segmented 

 worms, for it is entirely destitute of setae, and the convoluted 

 intestine opens to the exterior not far from the front end of the 

 body on the upper surface. 



WHEEL-ANIMALCULES (ROTIFERA or ROTATORIA) 



The Wheel - Animalcules are minute transparent animals 

 mostly found in fresh water, though some are marine, and others 

 are to be found on damp earth and vegetation. They present 

 great diversity in form and structure, and as microscopic objects 

 are unsurpassed for beauty and interest. Unfortunately their 

 true affinities are yet to seek, but as likely as not they are the 

 degenerate descendants of higher worms, and are to be looked 

 upon as permanent larvae, the original adult form having been 

 dropped out of the life-history. That such a thing is possible is 



shown by the case of the Axolotl among 

 Amphibia, to which reference has already 

 been made (p. 249). 



A common and beautiful fresh-water form, 

 which will serve as a type, is the Rose-coloured 

 Rotifer (Philodina roseola) (fig. 268). The 

 elongated body is covered by a firm cuticle, 

 and this is marked by transverse furrows so 

 as to give a deceptive appearance of seg- 

 mentation. The posterior end of the body 

 tapers into a jointed tail ending in a pair of 

 forceps, and by means of this region the 

 animal is able to progress somewhat like 

 a leech. When the rotifer is fully extended, 

 a couple of prominences are seen at the 

 front end which together constitute the wheel -organ. Each of 

 them is fringed with a circlet of cilia, and as these move one 

 after another in a very regular way a deceptive appearance of 

 rotation is produced, suggesting the movement of a wheel. By 

 means of this organ the animal is able to swim and it also sets 

 up currents by which food particles are brought to the mouth, 

 lying in a depression at the front end of the body. The mouth 



GIZZARD 



STOMACH 



1NTESTIN 



Fig. 268. Rose-coloured Rotifer 

 (Philodina roseola], greatly en- 

 larged 



