On the other hand, those species that have the power of travelling through the air or traversing the ocean are far less fixed in their habitat; and it results from this that the list of so-called “British Birds” receives accessions from time to time from stragglers which find their way from the European continent or Asia, or even across the Atlantic. Slight but permanent variations from the continental type may be recognised in many native British species, some of the most marked among vertebrated animals being the Irish Stoat, the Squirrel, the Red Grouse, the St. Kilda Wren, the Coal-Tit, the Goldcrest, and several species of fresh-water fishes, mostly belonging to the genera _Salmo_ and _Coregonus_. Some of the latter, such as the Vendace, the Gwyniad, and their allies, of which specimens are exhibited in the wall-case in the pavilion, have an extremely local distribution, being found only in certain small groups of mountain lakes. Of the Seals, only two species are really natives of Britain, the Common Seal (_Phoca vitulina_) and the great Grey Seal (_Halichœrus grypus_); specimens of both these are shown in the pavilion. Those desirous of studying more minutely the characteristics of British Mammals should examine the series of skins and skulls exhibited in a special case on the right side of the central west window. [Sidenote: Coral Gallery.[10]] Parallel with the Bird-gallery, on the north side (right on entering), and approached by several passages, is a long narrow gallery containing the collection of Corals and Sponges and allied types. Commencing at the eastern end, some of the lowest forms of animal life are exhibited in the wall-case and table-cases; they belong to a group called PROTOZOA, and, for the greater part, are so minute, that they can be studied only with the microscope; their structure is therefore illustrated chiefly by means of models and figures. The next divisions of the gallery are occupied by the Sponges, most conspicuous among these being a series showing the variations of the common Bath-Sponge (cases 1 and 2), the beautiful flinty Venus’ Flower-basket or _Euplectella_ (fig. 17), the Japanese Glass-rope Sponge or _Hyalonema_ (case 3), and the gigantic Neptune’s Cup or _Poterion_, of which several specimens are placed on separate stands. Special interest attaches to the case showing the different kinds of Sponges used in commerce. [Illustration: FIG. 17.--VENUS’ FLOWER-BASKET (_Euplectella imperialis_ and _E. aspergillum_). (One-sixth natural size.)] [Illustration: FIG. 18.--BRAIN-CORAL. (_Meandrina cerebriformis_).]