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.
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.
Fig. 61.—Malacobdella grossa O. F. Müll., a large female specimen. Kiel. × 1. (From von Kennel.) A, From the dorsal surface; B, from the ventral surface.
Palaeontology.—Nemertines are unknown in a fossil state; this is probably owing to the softness of their bodies, which would render their preservation extremely improbable.
Affinities.—Until recently the Nemertines were regarded as a sub-order of the Turbellaria. They were afterwards separated from the Turbellaria and placed as a distinct class of the phylum Platyhelminthes.
Some zoologists have considered them to be so different in many respects from the other classes of the Platyhelminthes as to justify their being altogether separated from that phylum, and treated as a distinct group.