The fact that the gills of the latter had been overlooked was readily explained by the circumstance that they are retractile, and not merely contractile. But all the species of the genus _Siphonogaster_, or _Alma_, as it ought really, following the rules of priority, to be called, have not got gills, as is the case too with the genus _Hesperodrilus_. The gills of _Alma_ are branched, and there is therefore no longer any justification whatever for defining the Oligochaeta as a group of Annelids without gills. The simple gill-like processes of _Chaetobranchus_ might have been held to be not accurately comparable to the more complex structures which we find in the marine worms. [Illustration: FIG. 189.—Transverse section through _Branchiura sowerbyi_. × 20. _d.br_, Dorsal branchia; _i_, intestine; _n_, nerve-cord; _v.br_, ventral branchia.] NERVOUS SYSTEM.—The central nervous system of the Oligochaeta is very uniform in its structure in the entire group. The only family which is at all anomalous is that of the Aphaneura. In _Aeolosoma_ there appears to be only a pair of cerebral ganglia, which retain the primitive position of these organs in being still in direct connexion with the epidermis. In all other Oligochaeta there are a pair of cerebral ganglia, {354}connected by a circumoesophageal commissure with a ventral ganglionated cord. From the cerebral ganglia arises a system of nerve-fibres and nerve-cells, which represents the stomatogastric nerves of other Invertebrates. SENSES AND SENSE-ORGANS.—The only organs that can be regarded with anything like probability as sense-organs are the pigmented eyes of certain Naids and the tactile cells of many worms. The latter are usually elongated cells provided at their free extremity with a stiff process; they occur associated in groups, and often these bundles of cells have a segmental arrangement. The head end of many of the lower Oligochaeta, for instance the genus _Aeolosoma_, has delicate processes projecting here and there; these appear to be also of a tactile nature, and are of course connected with cells of the epidermis. The eyes of certain Naids are little more than lenticular bodies embedded in a mass of pigment. In the genus _Eudrilus_ and in many Eudrilidae are peculiar integumental bodies, which were independently discovered by Dr. Horst[411] and myself, and compared by us to the Pacinian bodies of Mammals. Whether these structures are connected with nerves or not is doubtful. In spite of the poor development and the simplicity of their sense organs, the higher Oligochaeta at any rate can feel, and can distinguish light from darkness. Darwin[412] came to the conclusion that "light affects worms by its intensity and its duration." And furthermore, it is only the anterior end of the body which is thus affected. Of the sense of hearing these animals appear to be utterly devoid. Some kept by Darwin "took not the least notice of the shrill notes