1911 Encyclopædia Britannica/Desmoscolecida
|From Cambridge Natural History, vol. ii., “Worms,” &c., by permission of Macmillian & Co. Ltd.
Female Desmoscolex elongatus Panceri, ventral view. a, Ovary. (From Panceri.)
DESMOSCOLECIDA, a group of minute marine worm-like creatures. The body tapers towards each end and is marked by a number of well-defined ridges. These ridges resemble on a small scale those which surround the body of a Porocephalus (Linguatulida), and like them have no segmental significance. Their number varies in the different species. The head bears four setae, and some of the ridges bear a pair either dorsally or ventrally. The setae are movable. Two pigment spots between the fourth and fifth ridges are regarded as eyes. The Desmoscolecida move by looping their bodies like geometrid caterpillars or leeches, as well as by creeping on their setae. The mouth is terminal, and leads into a muscular oesophagus which opens into a straight intestine terminating in an anus, which is said to be dorsal in position. The sexes are distinct. The testis is single, and its duct opens into the intestine and is provided with two chitinous spicules. The ovary is also single, opening independently and anterior to the anus. The nervous system is as yet unknown.
There are several species. D. minutus Clap. has been met with in the English Channel. Others are D. nematoides Greef, D. adelphus Greef, D. chaetogaster Greef, D. elongatus Panceri, D. lanuginosa Panceri. Trichoderma oxycaudatum Greef is 0.3 mm. long, and is also a “ringed creature with long hair-like bristles.” The male has two spicules, and there is some doubt as to whether it should be placed with the Desmoscolecida or with the Nematoda. With regard to the systematic position of the group, it certainly comes nearest—especially in the structure of its reproductive organs—to the Nematoda. We still, however, are very ignorant of the internal anatomy of these forms, and until we know more it is impossible to arrive at a very definite conclusion as to their position in the animal kingdom.