CLASSIFICATION AND HABITS OF NEMERTEA. 201 
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 
cecum. 
e.g. Amphiporus, Drepanophorus, Tetrastemma. 
An isolated form, A/alacobdella, 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. 
eg. Lineus, Cerebratilus. 
Habits.—Most Nemertines are marine, creeping about 
in the mud, under stones, among seaweed, and the like; 
many, ¢.g. Cerebratulus, are able to swim; Felagonemertes 
and Planktonemertes are \eaf-like hyaline forms of pelagic 
habit ; two or three species of Prostoma live in fresh water ; 
seven species of Geonemertes are terrestrial; MJalacoddella 
and a few others live in the mantle-cavity of marine 
bivalves, and some others are found as commensals in 
Ascidians; Cephalothrix galathee 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. 
PuyLtumM NEMATOHELMINTHES 
Class Nematoda, e.g. Ascaride. 
Class Nematomorpha, Gordiide. 
Class Acanthocephala, e.g. Echinorhynchus. 
