TEREAD-WOBMS. 163 



lation very rare ; a metamorphosis in the Bendroccda, the 

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

 nocelis, Catenula, Mesostomum). Suborder 2. Dendrocala 

 (Planaria, Dendroccslum, 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- 

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

 tebrates ; or with direct development [PolystometB). Sub- 

 order 1. Distomem (Monostomum, Amphilina, Distomum, 

 Amphistomum). Suborder 2. PolystomecB (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 stroMla ; 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). 



Laboratory TTorA;.— 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. — Kematelmii^^thes {Round, Thread-worms). 



General Characters of Thread-worms.— These worms are 

 either free or parasitic ; examples of the former exist iu 

 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 met 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. 



