GLIDIXG-WORMS. 79 



Primitive Worms, however, simultaneously with the forma- 

 tion of the secondary germ-layers, these various functions 

 also began to be distributed to various organs, which detached 

 themselves from the original main organ, the primitive in- 

 testine. Special organs originated for reproduction (sexual 

 glands), for secretion (kidneys), for motion (muscles), and 

 for sensation (nerves and sense-organs). 



In order to obtain an approximate picture of the sim- 

 plest form in which all these various organs first appeared 

 in the Primitive Worms, it is only necessary to examine 

 the most imperfect forms of Gliding-worms (Turbellaria), as 

 they exist at the present time in salt and fresh water. They 

 are mostly very small and insignificant Worms of the simplest 

 form, many being scarcely 1 a millimetre or a few millimetres in 

 length. In the simplest species of Gliding- worms the greater 

 part of the oval body is occupied by the intestinal canal. 

 This is a very regularly shaped pouch with an opening, re- 

 presenting both mouth and anus (Fig. 184, m). At the 

 anterior section of the intestinal tube, which is separated 

 as a throat (pharynx, sd\ the fibrous layer is very thick, 

 a thick muscular layer. Immediately outside the intestinal- 

 fibrous layer lies the skin-fibrous layer, which in moet 

 worms appears as a large skin-muscle sac. Above the 

 throat in Gliding-worms a nerve system of the simplest 

 form is already visible in front, a pair of small nerve- 

 knots, or ganglia, which from their position are called the 

 " upper throat ganglia," or " brain " (Fig. 185, g). Delicate 

 nerve-threads (n) pass from this to the muscles and to the 

 ciliated skin-sensory layer. A pair of quite simple eyes 

 (au) and nose-pits (no) are to be found in a few Gliding- 

 worms. The Flat-worms are also universally provided with 



