26 INVERTEBRATE ZOOLOGY. 



parasitic forms. -^ Trichina. Angnillula 

 (vinegar-eel). Ascaris (pin-worm). Oor- 

 dins (hair-snake). 



Class V. ACANTHOCEPHALi A single family of worms, externally 



similar to last, but with an anterior ex- 

 tensile proboscis beset with hooks, and 

 no intestinal canal. The l)ody wall de- 

 velops two solid masses {Lemnisci) which 

 project into the protocoelom. The re- 

 productive organs open posteriorly. De- 

 velopment by metamorphosis. Larval 

 stage in crustaceans and insects, adult in 

 intestine of vertebrates. 

 ■^ Echinorhynchus. 



SUPPLEMENTARY CLASS — NEMERTINI. 



Flattened forms, often very long, externally covered with cilia. At ante- 

 rior end a long retractile proboscis. Protocoelom filled with parenchyma. 

 Alimentary canal complete vnth anal orifice. Nephi'idia and gonads repeated 

 in pairs along the body. Complicated blood and nervous systems. Bisexual. 

 Many develop by metamorphosis and possess a curious larval form, the 

 "Pilidium," which is similar to a Protrochula. 



The relations of this group are very imcertain. Many characteristics would 

 place them near the Platodes, but the complete alimentary canal, the blood 

 system and the segmentally arranged nephridia and gonads would exclude 

 them from these and place them nearer the Annelides. It seems at present 

 more probal)le that they are Protonephridozoa, and that tlie metameric char- 

 acter of some of the internal organs is due to a reduplication of protosomatic 

 elements, rather than to the formation of a nietasoma (cf . the reduplication of 

 the abdomen in Taenia). 



Order 1. Hoplonemertini Proboscis armed with bristles. 



Order 3. Schizonemertini Proboscis unarmed, head nearly divided 



by deep, longitudinal fissures. 



Order 3. Palaeonemertini Proboscis imarmed. Head nearly entire. 



