Fig. 181.—Sternaspis scutata Ranz. × 2. (From Vejdovsky.) The left figure shows the ventral surface; the right represents the internal organs as seen when the body-wall is pinned aside, having been slit up along its dorsal surface. a, Anus; c, gills; ch, anterior strong chaetae; ch1, bundles of chaetae along the lateral margin of the shield; ch2, the posterior marginal chaetae; d, oviduct; g, the external tube carrying genital pore; i, coiled intestine; n, nephridium ("brown tube"); o, ovary, amongst the coils of the alimentary canal; p, pharynx; pr, prostomium, with mouth just behind it; s, shield (on left figure); s, stomach (on right); sh, outline of shield seen through the ventral body-wall (in right figure); x, chaetae embedded in the body-wall, with nerves passing by them.
Fam. 6. Sternaspidae.—The single genus, Sternaspis, has not been recorded on our coasts, but is of so peculiar a structure as to deserve a description.[[391]] S. scutata Ranzani, occurring in the Mediterranean, is rather less than an inch in length, and derives its name from the possession of a pair of peculiar "horny" plates or shields on the ventral surface posteriorly. Around their margins are set about thirty bundles of long capillary chaetae. There are three half rings of stronger chaetae on each side near the anterior end of the body. The mouth is overhung by a very small rounded knob (prostomium), which in S. spinosa Sluiter, is prolonged outwards on each side to form a grooved palp-like organ, recalling that of Bonellia. The anus is placed posteriorly; and in front of it, on the dorsal surface, are two bundles of many long thread-like gills. On the ventro-lateral surface, in front of the middle of the body, is a pair of finger-shaped processes containing the genital ducts. The anterior segments of the body can be withdrawn into the body, as in the Chlorhaemidae. Further examination leads to the conclusion that the body of Sternaspis consists of about thirty segments, most of them provided with paired bundles of capillary (neuropodial?) chaetae, distributed as follows:—Each of the segments 2, 3, 4 has a half ring of strong chaetae on each side; segments 5 to 7 are without chaetae; segments 8 to 14 have chaetae embedded in the body-wall, but not projecting. The shields cover the remaining segments; and along the outer edge of each are some ten bundles of chaetae, and along the hinder edge some five or six bundles, representing as many segments. Thus the worm consists of about thirty segments whose outlines are nearly obliterated (as in the Chlorhaemidae), and whose chaetae, except those which are specially developed, are disappearing: while posteriorly a great shortening of the body brings the bundles close together. A continuation of this process, involving a further disappearance of chaetae, leads readily to the condition met with in Echiurus, one of the chaetigerous Gephyrea. Internally, further evidence of the relation between Sternaspis and the Chlorhaemids with the Gephyrea is afforded by the absence of septa, by the coiled alimentary canal, and by the presence of a single pair of nephridia, which in the latter group act both as excretory organs and as genital ducts.
BRANCH B. CRYPTOCEPHALA.
Sub-Order 1. Sabelliformia.[[392]]
Fam. 1. Sabellidae.—The branchial crown consists of a usually considerable number of filaments arising from a semicircular base. The peristomium may be reflexed to form a collar, which is frequently notched, so that a lateral and a ventral lobe on each side may be distinguished (Fig. 133, B, p. [261]). The thorax consists of nine segments, and is provided with ventral "gland shields," which are continued along the abdomen, where they are subdivided into two by a ciliated "faecal groove," which sometimes bends to one side on reaching the thorax, and may extend forwards along the dorsal surface to the head: this groove serves to carry the faeces out of the tube. The gill filaments are always provided with secondary processes, and may be provided with compound eyes.[[393]] The tubes of the Sabellidae are always of fine mud or of sand.
Sabella pavonia Sav. is about 10 to 12 inches long and about ¼ inch across; the tube of fine mud is considerably longer and embedded deeply in the mud, with its free end projecting to some 2 or more inches, where it serves for the attachment of seaweed, Polyzoa, Hydrozoa, etc. The colour of the animal is orange-brown; the gills, which are about 1½ inches long, are green (due to contained blood) marked with more or less extensive brown or purple-brown spots, which may even hide the green tint. There is a pair of dark filaments arising between the dorsalmost gill filaments, which have been erroneously regarded as "prostomial tentacles"; they are, in fact, prolongations of a peculiar membrane or lip round the base of the gills, which bounds a groove leading to the mouth. These lip-processes (Fig. 133, B, l) occur in other Sabellids. Atlantic, North Sea, and Mediterranean.
Fig. 182.—A, A gill filament of Branchiomma; B, of Dasychone. a, Axis; f, secondary filaments; o, eye; x, dorsal appendices.
Branchiomma vesiculosum Mont. forms a sandy tube near low tide-mark. The animal, measuring 6-7 inches, is rich brown, darker anteriorly, abundantly speckled with white; the ventral surface is pink; the gills are green or olive-brown, marked with white bands in a fairly regular fashion. Each gill filament has, just below its tip, a compound eye, consisting of several lenses and retinae. North Sea and Mediterranean.