The Parapodia of Arenicola marina L. (Polychæta)

Summary.1 The use of polarized light is recommended for working out the anatomy of the muscles. 2 A neuropodium consists of a single dorsi‐ventral row of sigmoid chætæ, each in its own follicle. There is a continual dorsalwards procession of chætæ and follicles along it. New ones are formed at the ventral end, and old ones break down into a green, granular mass at the dorsal. The green matter collects for a time and is then cast out. 3 The neuropodium is provided with protractor and retractor muscles. 4 A notopodium is more complicated than a neuropodium, but built on the same fundamental plan. It can be conceived as a neuropodium in which the plate formed by the chætæ and follicles has (i.) changed its shape, (ii.) split into two parallel plates in close apposition, (iii.) rolled up into a scroll, and (iv.) acquired an invaginable base, by thinning and protrusion of the surrounding body wall. Chætal destruction and replacement proceeds as in the case of the neuropodium. 5 The intricate musculature of the notopodium is fully described in the text; it includes muscles corresponding to those of the neuropodium, and others which appear to be derived from new components. 6 Apart from the movements of the notopodia and neuropodia, the more anterior chætigerous annuli can perform distinctive “flanging” movements of their own. 7 Besides bearing the appendages, the chætigerous annuli are distinguished by important peculiarities of their general structure. These are most clearly seen in the first three segments, where each chætigerous annulus has (i.) an extension of the body cavity, the “parapodial canal,” looping round inside it; (ii.) a special musculature associated with the parapodial canal; (iii.) an externally visible “hinge‐line,” roughly bisecting the annulus; and (iv.) another special musculature associated with the hinge‐line. 8 The generally held view that the parapodial base has disappeared in Arenicola , leaving the notopodium and neuropodium to arise separately from the body wall, is apparently not true of the first three chætigerous annuli. The whole of any one of these annuli can be regarded as derived from a pair of parapodial bases, which have expanded dorsally and ventrally to form a girdle encircling the worm. 9 The fourth to nineteenth chætigerous annuli are described as each consisting of (i.) a “parapodial component,” containing special structures like those seen in the first three, and (ii.) a “body‐wall component,” from which these peculiarities are absent. As one passes backwards, the body‐wall component comes to occupy more and more of the annulus, at the expense of the parapodial component. 10 The gills and nephridiopores are referred to the body‐wall component.