THREAD-WORMS. 121, 
lation very rare ; a metamorphosis in the Dendrocela, the 
larva being a trochosphere. Suborder 1. Rhabdocela (Mo- 
nocelis, Catenula, Mesostomum), Suborder 2. Dendrocela 
(Planaria, Dendrocelum, Geoplana, and Bipalium). 
Order 2. Trematoda.—Usually flat, oval, rarely cylindrical, not seg- 
mented, parasitic worms, with a mouth, forked intestine, 
no anus ; a large sucker near the middle of the body, or 
several smaller ones ; either with a metamorphosis (Dis- 
tomez), the larva living in mollusks, etc., the adult in ver- 
tebrates; or with direct development (Polystomee). Sub- 
order 1. Distomee (Monostomum, Amphilina, Distoraum, 
Amphistomum). Suborder 2. Polystomee (Aspidogaster, 
Diplozoon, Polystomum, Gyrodactylus). 
Order 3. Cestodes.—Parasitic, usually ribbon-like worms, without any 
mouth or digestive canal; with a nervous system, and an 
(excretory) water-vascular system; hermaphrodite, the 
joints (proglottis) numerous and containing male and fe- 
male reproductive organs ; the eggs minute and very nu- 
merous. The mature worm is many-jointed, the joints 
budding out from near the head ; in this form it is called 
a strobila ; the terminal joints fall off, becoming indepen- 
dent (proglottis). The eggs after fertilization pass through 
a morula and gastrula stage, a circle of hooks and suckers 
developing on the head (Caryophylleus, Tetrarhynchus, 
Ligula, Bothriocephalus, Teenia). 
Laboratory Work.—The flat worms have been most successfully 
studied by fine injections, especially by slicing hardened sections, 
which should be stained with carmine, and mounted for the micro- 
scope. 
Cuass II.—Nematetmintues (Round, Thread-worms). 
General Characters of Thread-worms.—These worms are 
either free or parasitic; examples of the former exist in 
abundance under stones, etc., between tide-marks, lying 
in coils; small, almost minute species occurring in fresh 
water and in damp earth, while the parasitic species, which 
are the most numerous, live free in the alimentary canal or 
imbedded in the flesh of their hosts, especially fishes and 
mammals. The species are remarkably persistent in form, 
