FLAT-WORMS. 141 



Cestoids especially, which have no body-cavity, as the Tar- 

 digrades or Linguatulce do to the higher Arachnida. 



Each species of Dicyema and Dicyemella (Fig. 92) com- 

 prises two sorts of individuals, differing externally, one (the 

 Nematogene) producing vermiform embryos, the other 

 (Rltombogene] infusoriform (but many-celled) young. The 

 Nematogenes produce germs which undergo total segmen- 

 tation, and assume a gastrula condition. After the closure 

 of the primitive opening, the body elongates, and the worm- 

 like form of the adult is finally attained, when it passes 

 through the body-walls of the parent. 



The germs of the Rhombogenes arise endogenously in 

 special cells lodged in the axial cell, and called " germi- 

 genes." The germ-like cells undergo segmentation, and 

 then form small spheres, which become infusoriform em- 

 bryos. The worm-like young is destined to be developed 

 and live in the Cephalopod where it has been born, while 

 the infusorian-like young probably performs the office of 

 disseminating the species. It is possible that in those ani- 

 mals, such as the Cetacea, which feed on cuttlefishes, these 

 worms (the Nematogenes at least) may pass into a genuine 

 vermian form. 



CLASS I. PLATYHELMINTHES (Flat-worms, Tape-worms, 



Fluke-worms, etc.] 



Order 1. Turbellaria. In any pond of standing water 

 one can find on the under side of sticks or stones, small 

 dark flat worms. These are Planarian 

 worms. The common dark-brown, 

 almost black Planar ia torva Miiller 

 (Fig. 93) is about six or eight milli- 

 metres long, oblong, flat, with two 

 black eye-spots, with an oblong oval pi g 93 Fi g 94 



space in front of each eye. A form 

 allied to this is a perfectly white Plana- 

 rian called Dendroceelum lacteum Oersted, which lives under 



