_Trophonia plumosa_ Müll. is about 2 to 4 inches long, yellowish-brown in colour, with a rough skin; the head is usually retracted. It lives in the mud amongst Laminarian roots down to 50 fathoms. North Atlantic. _Siphonostoma_ (_Flabelligera_) _diplochaitos_ Otto, has a transparent body-wall, so that the coloured viscera are visible. The skin carries long papillae, which traverse a thick jelly-like envelope secreted by it, in {335}which numerous diatoms live (symbiotically?); the surface is covered by particles of mud, etc. This species, which may be found under stones at low tide, occurs also in the Mediterranean. [Illustration: 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; _ch^1_, bundles of chaetae along the lateral margin of the shield; _ch^2_, 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 {336}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