CLASSIFICATION AND HABITS OF NEMERTEA. 201 



Order Mesonemertini. Lateral nerves in the muscular layer ; mouth 

 behind brain ; no stilets. 



Carinoma, Cephalothrix. 



Order Metanemertini. 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. Amphiporus, Drepanophorus^ Tetrastemma. 



An isolated form, Malacobdella, parasitic in bivalves, 

 has a posterior sucker, a coiled intestine, and other 

 peculiarities. 



Order Heteronemertini. Mouth behind brain ; no stilets ; three 

 layers of muscle, the outermost and innermost longitudinal ; 

 lateral nerves outside circular muscular layer. 

 e.g. Limits, Cerebratuhis. 



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 ; two or three species of Prostoma 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. 

 The fresh-water Prostoma lumbricoides forms a protective 

 cyst of mucous threads in unfavourable conditions, and 

 Tetrastemma dorsale often does the same along stems of 

 the hydroid Tubularia. 



PHYLUM NEMATOHELMINTHES 



Class Nematoda, e.g. Ascaridas. 



Class Nematomorpha, Gordiidse. 



Class Acanthocephala, e.g. Echinorhynchus. 



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



The Nematodes are unsegmentcd, more or less thread like 

 " worms" some free-living and others parasitic. The body is 



