FLAT-WORMB. 141 



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

 digrades or Linguatulm do to the higher Arachnida. 



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

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

 Nematogene) producing vermiform embryos, the other 

 {Rhombogene) inftisoriform (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 inf usoriform 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 Ifematogenes at least) may pass into a genuine 

 vermian form. 



Class I. — Platthelminthes {Flat-worms, Tape-ivorms, 

 FluTce-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 Planaria torva Miiller 

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

 metres long, oblong, flat, with two 

 black eye-spots, with an oblong oval pig. 93. rig. 94. 



space m front ot each eye. A lorm ^^^^ percamim. 

 allied to this is a perfectly white Plana- 

 rian called Dendroccelum lacteum Oersted, which lives under 



I 



