Development of Terebella nebulosa. (After Milne Edwards.)

Fig. 136. Development of Terebella nebulosa. (After Milne-Edwards).

(711). Having become deprived of the locomotive cilia with which they were previously furnished, the larvae now cease swimming and begin to enclose themselves in a kind of mucous substance, which gradually solidifies, so as to form a cylindrical tube open at both extremities. The first period of their existence, during which they lead an erratic life, then closes, and they begin to assume the habits of their parents. The ventral oars, with their armature of terminal hooklets, are successively developed in a regular series from before backwards, as additional segments are added to the length of the body. The tentacular appendages next begin to be developed from the sides of the head. But it is not before the body has acquired thirty-eight or forty pairs of feet that the branchial apparatus makes its appearance, under the form of two simple tubercles developed from the lateral regions of the neck; these, however, rapidly enlarge, and soon assume the functions which, in the adult animal, they are destined to perform.

(712). The structure of the nervous system in the Annelida conforms, in its arrangement, with the general type common to the articulated classes. A considerable supra-cesophageal mass (fig. 137, a a) represents the encephalon, in front of which are situated minute ganglia (b, c, d, e), from whence nerves are derived to supply the principal instruments of sensation connected with the cephalic portion of the animal. The circum-oesophageal ring (nn) is strongly marked, communicating on each side with the ventral series of ganglia (o, p) that extends throughout the entire length of the body, giving off nerves to supply the different segments. Communicating with the posterior aspect of the encephalic ganglia are several small ganglionic masses (i, k, I, m), which are joined together by delicate filaments, and apparently represent the sympathetic system, inasmuch as from them are derived filaments supplying the alimentary canal and the principal viscera.

Plan of the nervous system in the Dorsibranchiate Annelidans.

Fig. 137. Plan of the nervous system in the Dorsibranchiate Annelidans. (After Quatrefages).

(713). In Torrea vitrea (an Annelid the transparency of which is such that, when plunged into sea-water, its presence is only distinguishable from the bright red colour of its eyes and a double line of violet-coloured spots that extend along its back), M. de Quatrefages* was enabled to examine the structure of the organs of vision in a very satisfactory manner. The eyes in this species are only two in number; and, indeed, they constitute by far the larger part of the creature's head, forming two very considerable prominences that are almost conjoined in the mesial line of the body. The integument, which is here extremely thin and perfectly diaphanous, passes over the ocular globe, and evidently in this case performs the functions of a transparent cornea (fig. 138,1, a.) A thick fibrous stratum, representing the sclerotic (d), encloses the eye, and becomes continuous with the sheath, likewise fibrous (h), of the optic nerve (g'.) The colourless sclerotic presents upon one side a large irregularly-rounded aperture that is partly closed by a sort of choroid of a brownish colour (b), in the centre of which is an almost circular pupil surrounded with a border of a deep blue colour. Through the pupillary opening it may be perceived that the interior of the eye is lined by the choroid, and that the whole interior of the ocular capsule is filled up with a vitreous humour so absolutely transparent that the crystalline lens situated in its centre seems to be in connexion with nothing. On the outside of the eye the optic nerve can be plainly seen arriving at the eyeball and expanding to form the retina. The eyes of other Annelidans are, however, when present, by no means so easily examined; they may, however, from the researches of Miiller*, "Wagner1, Rathke2, and Siebold§, be briefly stated to consist of a round transparent medium or lens enclosed in a layer of pigment, and provided posteriorly with a retinal expansion.

Structure of the eye in Torrea vitrea, and of the supposed auditory apparatus in Arenicola (after Quatrefages.)

Fig. 138. Structure of the eye in Torrea vitrea, and of the supposed auditory apparatus in Arenicola (after Quatrefages.) 1. a a, integument passing in front of the eye, and forming a transparent cornea; b, c, granular cellular tissue enclosing the globe of the eye; d, external surface of reticular pigmental membrane; f, internal surface of the same, seen through the pupillary aperture; e, the iris; g, the crystalline lens; g\ optic nerve; h, sheath of ditto, derived from the dura mater; i, k, vascular trunks forming a circle around the base of the eyeball. 2. Auditory apparatus of an Arenicola: a, acoustic nerve; b, c, cellular tissue investing the auditory capsule; d, otolithic masses. 3. Auditory apparatus of Amphicoryne:, a, cellular tissue; b, auditory capsule; c, otolith.

* Ann. des Sci. Nat. 1850.

(714). An apparatus to which the functions of an organ of hearing have been attributed by several eminent anatomists is met with in some Annelidans. Grube and Stannius || first announced a very remarkable structure in Arenicola, the existence of which has been confirmed by subsequent observers, that certainly resembles very closely in its conformation an organ common among the Mollusca, to which a similar function has been generally conceded:. this consists of a transparent membranous capsule (fig. 138, 2 & 3, a, b, c) enclosing a fluid, wherein one, or sometimes several minute bodies, having every appearance of otoliths, are suspended. M. de Quatrefages describes these auditory capsules as being situated in the first or second segment of the body, one on each side of the opening of the oesophagus, and observes that a nerve of considerable size is distinctly traceable in them.