RIBBON-WORMS 235 



Classification. 



Order Protonemertini. Brain and lateral nerves outside the 

 muscular layers ; mouth behind brain ; no stilets. 

 Carinella, Hubrechtia. 



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. Lineus, Cerebratulus.^ 



Habits. — Most Nemertines are marine, creeping about 

 in the mud, under stones, among seaweed, and the hke ; 

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

 and Planktonemertes are leaf-Hke 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 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 irri- 

 tated, and some are able to regenerate what they lose in this 

 way. The fresh-water Prostoma lumbricoides forms a pro- 

 tective cyst of mucous threads in unfavourable conditions, 

 and Tetrastemma dorsale often does the same along stems of 

 the hydroid Tubularia. 



