[Illustration: FIG. 183.—A group of tubes of _Serpula vermicularis_ L., from the mouths of two of which the animals are protruding, that on the right being seen from above. _br_, The gill plume; _m_, thoracic membrane; _op_, operculum; _op^1_, corresponding gill filament of the opposite side; _t_, tube. (From Cuvier's _Règne Animal_.) Nat. size.] _Filigrana implexa_ Berkeley is a small worm, but the slender white tubes intertwine and adhere together in masses 3 or 4 inches high, occurring at low tide and down to 18 fathoms in the North Sea and Mediterranean. The animal has only eight gill filaments on each side, one of which on each side is slightly expanded to serve as an operculum. The worm multiplies by transverse division. _Spirorbis borealis_ Daudin is a still smaller worm, the tube of which is coiled in a flat spiral about 1/16 to ⅛ inch across; it is {341}common, adhering to _Fucus_, shells, and other objects. It is represented by fossils in the Palaeozoic rocks. Cosmopolitan. _Protula_ (_Psygmobranchus_) _tubularia_ Mont. is a Serpulid without an operculum; it forms a straight or slightly and irregularly curved tube. Atlantic and Mediterranean. _Salmacina dysteri_ Huxley has no operculum; it is a small worm incrusting seaweeds, or forming masses like _Filigrana_. SUB-ORDER 2. HERMELLIFORMIA. The single family _Hermellidae_ is represented by two species—_Sabellaria alveolata_ L., which is littoral, and _S. spinulosa_ Leuck., occurring in 10 to 30 fathoms. _S. alveolata_[396] is about an inch long; the thorax is purple, the abdomen yellow to red. The narrow caudal region is bent sharply forwards, so that the anus, situated at its tip, comes to lie at the orifice of the tube, which is irregular and sandy. Great numbers of the animals live together, so that the masses of their tubes may be 2 or 3 feet thick and several feet long. They are well seen on the shore, at Paignton, near Torquay, and on Hilbre Island, off the Cheshire coast. North Sea, Atlantic, Mediterranean. [Illustration: FIG. 184.—_Spirorbis._ A, the shell, enlarged. B, the animal, × 50. _c_, Peristomial collar; _e_, eggs in the brood pouch at the end of the operculum; _g_, gills; _m_, thoracic membrane (characteristic of Serpulidae); _n_, the single pair of thoracic nephridia opening by a median dorsal pore beneath the collar (common to all Sabelliformia); _o_, ova in the anterior abdominal segments; _op_, operculum; _s_, spermatozoa in the hinder abdominal segments; _st_, stomach. (From Claparède.)] ORDER III. MYZOSTOMARIA.[397] These animals are parasitic on Crinoids or Asteroids.[398] The {342}single family, Myzostomatidae, contains but two genera, _Myzostoma_ F. S. Lkt. and _Stelechopus_ v. Gr.