Pint glass small pits which aid in nucleation, allowing the gas within it to be released more easily, thus preserving the head. Without the aid of these pits a regular pint glass will keep a head for only 3 or 4 minutes before appearing 'flat'. The markings come in a variety of styles ranging from a simple circular or square hatched pattern to more complicated branding messages. Pint glass collecting is an increasingly popular way for individuals to commemorate their visits to popular tourist destinations, most notably to microbreweries or sports arenas. These destinations often sell pint glasses adorned with their logos,
External gills salamander species, such as most members of the Proteidae family (the olm and mudpuppies) and the Sirenidae family, which naturally never metamorphose into an air-breathing form. The embryos of frogs and caecilians also develop external gills at some point in their development, though these are either resorbed before or disappear shortly after hatching. Fossils of the distant relatives of modern amphibians, such as "Branchiosaurus" and "Apateon", also show evidence of external gills. The external gills commonly consist of a single stalk (rami) protruding from a gill arch behind the head of the animal, above an associated gill slit. The stalk