This habit of passing the dry season in the hardened mud forming the bottom of exhausted pools and rivers is practised by several species of tropical freshwater fishes, belonging principally to the family _Siluridae_. The members of this group are able to exist and thrive in moist mud, and can even support life for a comparatively long time out of water altogether. The instinct is exhibited by species occurring both in the eastern and western hemispheres, as is shown by its record in the case of species of _Callicthys_ and _Loricaria_ in Guiana and by _Clarias lazera_ in Senegambia. It is also met with, according to Tennent, in a species of climbing perch (_Anabas oligolepis_) found in Ceylon and belonging to the family _Anabantidae_, all the species of which are able to live for a certain length of time out of water, and may sometimes be found crawling across land in search of fresh pools. The habit is also common to some species of mud fishes of the order Dipneusti, in which the air bladder plays the part of lungs. _Protopterus_, from tropical Africa, for instance, burrows into the mud and remains for nearly half the year coiled up at the bottom in a slightly enlarged chamber. The walls of this are lined with a layer of slime secreted from the fish's skin, and the orifice is closed with a lid the centre of which is perforated and forms an inturned tube by means of which air is conducted to the fish's mouth. The aestivating burrow of the Brazilian mudfish (_Lepidosiren_) is similar, except that the lid is perforated with several apertures. The Australian mudfish (_Ceratodus_) is not known to hibernate or aestivate. In countries where winter frosts arrest the growth of vegetation terrestrial mollusca seek hibernacula beneath stones or fallen tree trunks, in rock crannies, holes in walls, in heaps of dead leaves, in moss or under the soil, and remain quiescent until the coming of spring. Amongst pulmonate gastropods, most species of snails (_Helix_, _Clausilia_) close the mouth of the shell at this period with a membranous or calcified plate, the epiphragm. Slugs (_Limax_, _Arion_), on the contrary, lie buried in the earth encysted in a coating of slime. Similarly in the tropics members of this group, such as _Achatina_ in tropical Africa and _Orthalicus_ in Brazil, aestivate during the dry season, the epiphragm preserving them against desiccation; and examples of two species of _Achatina_ from east and west Africa exhibited in the Zoological Gardens in London remained concealed in their shells during the winter, although kept in an artificially warmed house, and resumed their activity in the summer.