THE ANNELIDAN WORMS, OR ANNELIDES (ANNELIDA).

BY W. BAIRD, M.D., F.R.S., F.L.S., ETC.

FIRST ORDER.-ERRANTIA.

(With a Coloured Plate.)

FAMILY OF APHRODISEANS, SEA MICE. The Aphrodiseans (Aphroditida), though, perhaps, not the type of the free wandering Annelides (Annelida errantia), are usually the first that authors describe. Quatrefages considers them as the aberrant group of this order, though, at the same time, he commences his arrangement with them. The greater number differ in form from the bulk of the wandering Annelides; for, instead of being long and slender like the Nereids and Phyllodoces, for instance (Plate III., Figs. 3 and 4), they are generally short, rather broad, and more or less oval in shape. The chief character, however, is the possession of a certain number of membranous organs, like scales, which have been described by Savigny as resembling the wing cases of beetles, and hence, by him called elytra, a name adopted by most authors since his time. These scales, or elytra, are fixed to the base of the superior branch of the feet by means of a short peduncle, and lie along the back in a double series, covering it like a coat of mail (Plate III., Fig. 1). They are composed of two cutaneous layers, applied one against the other, but capable of being separated so as to become vesicular. At certain seasons of the year these vesicular scales are filled with ova; they are thus useful in the reproductive system, and, as I have previously mentioned, serve also as organs for aërating the blood. They are various in number and position, and do not occur on all the feet. Generally, they alternate with the cirri; those feet which possess elytra being deprived of cirri, whilst those which do not possess scales are provided with these organs.* The presence of feet, furnished with scales which alternate with other feet destitute of these appendages, is one of the most important characters of the group.

In our sketch of the respiratory system of the Annelides we have taken particular notice of the branchiæ. In many they are well developed, and attract attention by their peculiar form. In the Aphrodites, however, these organs are very small, are hidden under the scales, or elytra, and merely present the appearance of small cutaneous eminences, like minute nipples. In some of the * An exception to this rule is that of the genus Sigalion. VOL. III.-NO. 111.

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