A form of which we have a full description is _Tetrastemma aquarum dulcium_, found by Silliman[147] at Monroe, under stones in brooks in company with Planarians. It is a small worm of a red or pink colour, about half an inch in length, and it possesses usually three pairs of eyes. The proboscis is armed, and opens by a separate aperture. The excretory system consists of a vessel on each side of the body, each opening externally by a pore, and internally dividing into numerous branches which end in ciliated expansions. An individual of the same species was found by Beddard in one of the tanks in the Botanical Gardens in Regent's Park, but as the tank is one in which tropical plants are grown, it had almost certainly been introduced among the roots of the plants, and cannot be considered as a British species. A fresh-water Nemertine belonging to the genus _Tetrastemma_ was, however, found by Benham[148] on the roots of some water plants in the Cherwell at Oxford. The specimen was of a bright orange colour and measured half an inch in length. Du Plessis[149] found another fresh-water form on the lower surface of stones in shallow pools on the shores of the Lake of Geneva, and named it _Tetrastemma lacustre_. It is a small animal, the largest specimens being rather over an inch in length. Another European genus was found in 1893 by F. E. Schulze in Berlin. It has been fully described by T. H. Montgomery,[150] who has given it the name of _Stichostemma eilhardii_. {119}PARASITIC FORMS.—The genus _Malacobdella_ was found by von Kennel[151] in large numbers living on _Cyprina islandica_, a Lamellibranch Mollusc, in the harbour at Kiel; and it has also been described by Riches[152] as a British form. It is attached to its host by means of a large round sucker situated at the posterior end of the ventral surface, while the rest of the body waves about freely in the mantle-cavity. It is perhaps hardly correct to describe this animal as parasitic, since it does not appear to obtain its nutriment at the expense of the host by preying on its juices. The advantage of its position is, however, obvious, since a perpetual current of water is kept up in the mantle-cavity of the Mollusc, and from the stream the Nemertine is able to pick out and take for itself any food material which it considers suitable. At the same time it is not subjected to the influence of the winds and waves, as the shell of the mollusc acts as a barrier to prevent the entrance of disturbing elements. _Malacobdella_ is short and broad, somewhat flattened dorso-ventrally. The anterior end is bluntly rounded. The mouth opens into a wide pharynx, which is constricted behind and then passes into the intestine; this after a few coils opens by the anus situated dorsally immediately above the sucker. The proboscis opens into the pharynx.