78 ZOOLOGY. 



is a simple straight tube extending from the mouth to 

 the anus." 



" A single oval ganglion lies in the abdomen, and sends 

 forwards and backwards two pairs of lateral cords. The 

 anterior cords unite in front and above the mouth into a 

 hexagonal ganglion. This gives off two branches, which 

 dilate at their extremities into the spheroidal ganglia, on 

 which the darkly pigmented imperfect eye rests." 



SUB-KINGDOM III. Annuloida. 

 CLASS 1. Scolecida. 



107. The class Scolecida (Gr. scolex, a worm) includes the 

 intestinal-worms (entozoa), the microscopic wheel-ani- 

 malcules, the hair-worms, &c. These groups differ much 

 in external appearance, but there are some important points 

 of structure common to them all. Most of them possess 

 what is called the water vascular system, a series of 

 vessels which communicate with the exterior by one or 

 more openings on the surface of the body, their branches 

 permeating the interior. These vessels are believed to 

 perform the double functions of circulation and respiration. 



No heart or true circulatory apparatus has been ob- 

 served in any of them. 



108. The nervous system consists of only one or two 

 ganglia. 



109. There are seven groups included in this class 

 the Eotifera (or wheel-animalcules), the Turbellaria, the 

 Trematoda (or flukes), the Tseniada (or tape-worms), tho 

 Nematoidea (or thread-worms), the Acantocephala, and 

 the Gordiaceae. 



110. Rotifera (Lat. rota, a wheel fero, I carry). The 

 Kotifera are microscopic animals, commonly met with in 

 fresh water ponds, and infusions of vegetable matter, 

 which have been exposed to the sun for a few days. 

 They are very small, seldom exceeding the ^ of an 

 inch in length. Their true position in the animal 

 scale is still a matter of doubb. They were formerly 



