174 UNSEGMENTED "WORMS." 



Classification. — 



1. Palseonemertea : No deep head fissure ; no stilet ; mouth behind 



brain. 



e.g. Carinella, Cephalothrix, Carinoma, Polia. 



2. Schizonemertea : A deep head fissure with a ciliated duct to the 



brain ; lateral nerves between the longitudinal and inner circular 

 muscles ; mouth behind brain. 



e.g. Lineus, Cerebratuhts , Langia. 



3. Hoplonemertea : No deep head fissures ; lateral nerves inside the 



muscles ; stilet present ; mouth generally in front of brain. 

 e.g. Amphiporas, Nemertes, Drepanophorus, Malacobdella. 



The last has no head fissures nor spines on the pro- 

 boscis, but bears a posterior sucker. 



Habits. — Most Nemertines are marine, creeping about in the mud, 

 under stones, among seaweed, and the like ; many, e.g. Cerebrattilus, 

 are able to swim ; Pelagonemertes is pelagic ; four species of Tetra- 

 stemma live in fresh water ; seven species of Geonemertes are terrestrial ; 

 Malacobdella and a few others live in the mantle-cavity of marine 

 bivalves, and some others are found as commensals in Ascidians ; 

 Cephalothrix Galathece destroys the eggs of its host— the crustacean 

 Galathea. Most seem to be carnivorous, eating annelids, molluscs, and 

 even small crustaceans. Many break readily into pieces when irritated, 

 and the Schizonemertea are able to regenerate what they lose in this 

 way. 



Series Nematohelminthes. 



Class Nematoda, e.g. Ascaridae. 



Class Gordiacea, e.g. Gordiidae, Nectonemidas. 



Class Acanthocephala, e.g. Echinorhynchus. 



Class Nematoda. Thread-worms, Hair-worms, etc. 



The Nematodes are unsegmented, more or less thread-like 

 " worms," some of which are free-living and others parasitic. 

 The body is covered by a cuticle, often thick ; cilia are totally 

 absent, and the muscular system is very peculiar. From a 

 nerve-ring around the gullet, six nerves go forwards and six 

 backwards. The alimentary canal is usually well developed, 

 has mouth and anus, and is divided into three regions. 

 Vascular and respiratory systems are unrepresented ; there is 

 a distinct body cavity which is not cozlomic, and the remarkable 

 excretory system consists of two lateral canals opening to the 

 exterior by a single pore. The sexes are usually separate 

 and the organs simple ; there is distinct sexual dimorphism. 

 The life history is ofte?i intricate. 



