[Sidenote: Sea-Mats, Lamp-shells, and Sea-squirts.] In the same gallery are exhibited examples of three other groups of marine organisms, respectively known as Polyzoa, Brachiopoda, and Tunicata. The Polyzoa live in colonies, and include the so-called Sea-Mat. They are often mistaken for sea-weeds, although in reality they are animals of comparatively high organisation. The Brachiopods have two shells like Bivalve Molluscs, but the valves are dorsal and ventral--that is to say, back and front--instead of right and left. One valve is frequently perforated, hence the name of “Lamp-shells,” from a resemblance to an ancient Roman lamp. Sea-squirts, Tunicates, or Ascidians, are worthy of the attention of the visitor on account of their affinity with the Vertebrate stock, of which they may be regarded as a degenerate type. It is in the larvæ or “Ascidian Tadpoles” that the evidence of Vertebrate relationship is most conspicuous, as shown by certain important characters in the nervous system, skeleton, and respiratory organs. A series of large shells occupies some of the space in the wall-cases, among which attention may be directed to the thick and often handsomely coloured Helmet-shells (_Cassis_), which, together with the pink Queen-conch (_Strombus_), were formerly largely used in the now nearly obsolete art of cameo-cutting. In addition to these, a selection of British shells, and series of the eggs of Molluscs, as well as specimens illustrating the formation of pearls, and other points of special interest connected with Molluscs, are displayed. These specimens include not only the Pearl-oyster, but some of the other shells used in the mother-of-pearl trade. WHALE-ROOM. [Sidenote: Whale-Room.] Approached by a staircase, leading down from the last (or western-most) of the passages which connect the Bird-gallery with the Coral-gallery, is a separate room in which are placed the specimens of Whales and their relatives. For these, on account of their large size, no other place could be found in the Museum; but the room has, unfortunately, the disadvantage of being too small to display such large animals to full advantage. It is also intersected by columns, which interfere with the complete view of the larger specimens. As it is almost impracticable to preserve the skins of the larger species of Whales, owing to the oil with which they are saturated, the exhibition of the characters of these animals is carried out by means of their skeletons and artificial models of one side of the body. Complete models, which are much better than actual skins, of many of the smaller kinds, are shown. A general account of the structure and classification of the Cetacea, as Whales are technically termed, with reference to those exhibited in this gallery, will be found in a Special Guide.[16] [Sidenote: Fresh-water Dolphins.] [Sidenote: Narwhal.]