Gephyrea

Gephyrea, a class of worms, of interest owing to the many theories that have been suggested as to their true affinities. They are cylindrical or almost so, and are not marked off into distinct rings or segments like the earth-worms, etc. The anterior part of the body is provided with tentacles, or is prolonged into an extensile proboscis (prostomium); in either case the whole front of the body may be withdrawn into the animal. There may be a pair of hair-like seta? at the anterior end of the body, and in some cases also one or two rings of setae. The mouth is situated at the base of the proboscis, and the anus opens either at the extreme hinder end of the animal or on the dorsal aspect of the body near the anterior end. The nervous system consists of a band round the pharynx, from which a ventral cord runs backward through the body. There is a large body cavity (ccelome), filled with a fluid containing many small corpuscles. Nephridia are present, and may serve as the generative ducts. The sexes are distinct. They are all marine. The class is divided into two groups - the Gephyrea chactifera, which have a pair of setse and the mouth at the posterior end of the body, and the 67. achceta, in which the mouth is at the extreme anterior end, and the adult has no setae. The latter group is divided into two families - the Sipunculidcc and Priapulidtc; the former has a pair of nephridia, which serve as the genital ducts, while the anus opens on the dorsal side of the animal; in the latter the anus is nearly terminal. The affinities of the Gephyrea have been much discussed, and they have been classed with the Echinoderms through the resemblance of Sipunculus to the Holothurians (Sea-cucumbers) and the possession of a water vascular system. By another school they have been allied to the Bryozoa from the resemblance of some of the 67. aclueta to Phoronis. [Bryozoa.] Later researches have, however, shown that the class must be regarded as a degenerate group of worms, in which the normal segmentation has been lost; this is shown by the traces of segmentation represented by the four anterior pairs and one posterior pair of nephridia in some of the Chcetifera and the rudimentary setae; but with which division of the worms they must be most closely associated is still doubtful. Thus the males of Bonellia are minute and much like mature Planarians, whereas the characters of Echiurus ally it to the Chtetopods (q.v.); the family of the Sternaspida may be placed either among the Chaetopods or the Gephyrea. It is probable that several forms now included in this class must be separated from it, and the remainder may then be regarded as a group of degraded worms, allied most closely to the Chaetopoda (q.v.).