NEMA70DA. 189 



Order Iloplonemertea. Mouth in front of brain, usually opening 

 along with proboscis ; usually with stilets ; lateral nerves 

 internal to the muscular layers ; usually with an intestinal 

 caecum. 



e.g. Ainphiponts, Drepanophorns, Tetrastemma. 



An isolated form, Malacobdella, parasitic in bivalves, 

 has a posterior sucker, a coiled intestine, and other 

 peculiarities. 



Habits. Most Nemertines are marine, creeping about in 

 the mud, under stones, among seaweed, and the like ; 

 many, e.g. Cerebratulus, are able to swim ; Pelagonemertes 

 and Planktonemertes are leaf-like hyaline forms of pelagic 

 habit ; four species of Tetrastemma 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 galathea 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 some are able to regenerate what they lose in this 

 way. 



PHYLUM NEMATOHELMINTHES. 



Class Nematoda, e.g. Ascarida^. 



Class Nematomorpha, Gordiidre. 



Class Acanthocephala, e.g. Echinorhynchus. 



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



The Nematodes are tin-segmented, more or less thread-like 

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

 The body is covered by a cuticle, often thick, often subject to 

 moulting ; the muscular system consists of elongated muscle- 

 cells arranged longitudinally, and usually leaving two free 

 "lateral lines." 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 ; the cavity of the body is not cii'lomic ; the 

 remarkable excretory system consists of two lateral canals 



