16 THE ESSEX NATURALIST
tongues, these animals have undergone a series of adaptations
of structure which have enabled them to extend into every
type of habitat, whether rock, sand or mud, pelagic or
terrestrial. They are excluded only from the air (since their
structure, while permitting the development of fins, precludes
wings) and from arid areas of the earth due to their
susceptibility to desiccation.
An early and persistent line is that class which we call the
Placophora or Loricata (Chitons or Coat-of mail shells, Pl. 1
(1)). These animals, sharing the rocky shores with the limpets
(Pl. 1(2)), are distinguished from the latter by not having
undergone torsion, and by possessing eight articulated shell
plates on their backs.
Another untorted and highly successful class is that of the
bivalve molluscs or Lamellibranchiata (Fig. 4, and Plates
1, 2 and 3). In this group the shell is divided into two more
or less symmetrical valves, the gills are enormously enlarged
to function as food-collecting organs and the head is much
reduced in importance.
Fig. 4. Diagram of Unio, a freshwater mussel, with left shell valve and
mantle lobe removed, ex. exhalant, siphon: in, inhalant siphon: f, foot: g,
gill: m, mouth: ma, right lobe of mantle: p, palps. re, rejected material: sh,
right shell valve. Heavy arrows, inhalant and exhalant currents. Continuous
arrows on and below gill, rejection currents. Broken arrows on gill, feeding
currents.
All molluscs make great use of mucus for entrapping the
particles which might clog the gills, the material then being
moved off the gills and out of the mantle cavity by ciliary cur-
rents. Since a high proportion of the intruding particles is