THREAD- WORMS. 163 



lation very rare ; a metamorphosis ia the Dendroccda, the 

 larva being a trochosphere. Suborder 1. Rlmbdoccda (Mo- 

 nocelis, Catenula, Mesostomum). Suborder 3. Dendrocmla 

 (Planaria, Dendrocoelum, Geoplana, and Bipalium). 



Order 3. 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- 

 toroese), the larva living in moUusks, etc., the adult in ver- 

 tebrates ; or with direct development {Polystomece). Sub- 

 order 1. DistomecB (Monostomum, Amphillna, Distomum, 

 Amphistomum). Suborder 2. Polysiomem (Aspidogaster, 

 Diplozoon, Polystomum, Gyrodactylus). 



Order 3. Oestodes. — 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 (Caryophyllseus, Tetrarhynchus, 

 Ligula, Bothriocephalus, Taenia). 



LahmaUyry TFor/.;.— 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. 



Class II.— Nematblminthes {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 mc^t 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, 



