THREAD-WORMS. 



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lation very rare ; a metamorphosis in the Dendroccda, the 

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

 nocelis, Catenula, Mesostomum). Suborder 2. Dendroccela 

 (Planaria, Dendrocceluin, 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- 

 tomese), the larva living in mollusks, etc., the adult in ver- 

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

 order 1. DistomecB (Monostomum, Amphilina, Distomum, 

 Amphistomum). Suborder 2. PolystomecB (Aspidogaster, 

 Diplozoon, Polystomum, Gyrodactylus). 



Order 3. Cestodes. Parasitic, usually ribbon-like worms, without any 

 mouth, digestive canal, or nervous system ; with an (ex- 

 cretory) water-vascular system ; hermaphrodite, the joints 

 (proglottis) numerous and containing male and female re- 

 productive organs ; the eggs minute and very numerous. 

 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 independent (proglottis). 

 The eggs after fertilization pass through a morula and 

 gastrula stage, a circle of hooks and suckers developing on 

 the head (Caryophyllaeus, Tetrarhynchus, Ligula, Both- 

 riocephalus, Taeina). 



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. 



CLASS II. NEMATELMINTHES (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, 



