. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 386 AITNELIDA. fused with the mouth segment. Nais (Stylaria) proboscidca O. Fr. Mull. N. parasita Schm. Both species have a filiform prastotniurn. Chat ay aster vermicularis 0. Fr. Mull. Sub-class 2. — GEPHYREA.* Worms with cylindrical body, without external segmentation, ^v^th terminal or ventral mouth ; icith cerebral ganglion, ozsopliageal ring and ventral cord. Setce are sometimes present. The Gephyrea possess an elongated cylindrical body and live, as do the Holothuria, in sand and ooz
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. 386 AITNELIDA. fused with the mouth segment. Nais (Stylaria) proboscidca O. Fr. Mull. N. parasita Schm. Both species have a filiform prastotniurn. Chat ay aster vermicularis 0. Fr. Mull. Sub-class 2. — GEPHYREA.* Worms with cylindrical body, without external segmentation, ^v^th terminal or ventral mouth ; icith cerebral ganglion, ozsopliageal ring and ventral cord. Setce are sometimes present. The Gephyrea possess an elongated cylindrical body and live, as do the Holothuria, in sand and ooze in the sea. The characters which distinguish them as Annelids are the possession of an cesophageal ring connected with a cerebral ganglion and of a ventral cord par- tially surrounded by ganglion cells. The larvae of the Chce- ., tijera present traces ot seg- mentation (see below, p. 391), while in the Achxta the body- cavity remains simple. Of sense organs, eye spots have been observed ; these in certain Sipuncididce lie directly upon the brain ; there are also dermal papillae, into which nerves enter. The structure of the integu- ment is similar to that of the A nnelida ; the thick upper cuticular layer rests upon a _ cellular matrix, and is not un- PIG. 3ii.— Young Eehiurus from the ventral frequently wrinkled. There is side (after Hatschek) 0, Mouth at the base nQ external segmentation. The of the proboscis ; SC, cesophngeal commis. sure ; ss, ventral cord ; A, anus ; H, hooks, connective tissue dermis is of considerable thickness and en- closes numerous glandular tubes, which open to the exterior by pores in the epidermis. Belosv this is the strongly developed dermal muscular tunic, which is regularly composed of an outer layer of circular fibres. * Quatrefnges," Mc-moire sur 1'Echiure," Ann. des Sc. 3 S<' Tom VII. Lacaze-Duthiers, " Rccherches sur 1» Bonellia," Ann. <!rx Sr. Nat., 1858. \V. Keferstein, " Beitriige zur anatomis
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