FLAT- WORMS, 99 
Cestoids especially, which have no body-cavity, as the Tar- 
digrades or Linguatule do to the higher Arachnida. 
Each species of Dicyema and Dicyemella (Fig. 62) com- 
prises two sorts of individuals, differing externally, one (the 
Nematogene) producing vermiform embryos, the other 
(Rhombogene) 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. 
Crass L—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 Planaria torva Miller 
(Fig. 63) is about six or eight milli- | 
metres long, oblong, flat, with two 
black eye- -spots, with an oblong oval Fig. Bie. 64 
space in front of each eye. A form “prt ts 
allied to this is a perfectly white Plana- 
rian called Dendrocelum lacteum Oersted, oid lives under 
