MOLLUSCA.
The class Mollusca—pulpy animals—forms a grand division which man has been pleased to make in the animal kingdom, and immediately below the Vertebrata and above the Annulosa, which again stand above the Cœlenterata, which includes the polyps, sea-anemones, hydras, and corals, which last are more highly organized than the Protozoa.
Fig. 120. P. C. Mollusca.
The Mollusca may be divided into two groups, the Mollusca proper and the Molluscoïda. The mollusc proper, as represented in Fig. 120, presents the following parts, and is supposed to be bilaterally symmetrical, H, is the hæmal parts, in which the heart is situated, commonly called the dorsal part, although the word is used in a different sense in different divisions of the animal kingdom. In the same manner the opposite region (N) is not termed the ventral, but the neural part, in philosophical anatomy. It is the region in which the great centres of the nervous system are placed. The termination (a) is the anterior or oval part; the other end (b), the posterior or anal part: between these extremities the intestines take a straight course. The neural surface is that upon which the majority of molluscs move, and by which they are supported, and it is commonly modified to subserve these purposes by the formation of a muscular expansion or disk, called the foot. Three regions, in many genera very distinctly divided from one another, may be distinguished in this foot: an anterior, the Propodium (p p); a middle, the Mesopodium (m s); and a posterior, the Metapodium (m t). In addition to these, the upper part of the foot, or middle portion of the body, may be prolonged into a muscular enlargement on each side, just below the junction of the hæmal with the neural region, the Epipodium (e p). The mass of the body between the foot proper and the part of the abdomen which bears the epipodium may be termed the mid-body, or Mesosoma. On the upper part of the sides of the head are two pairs of organs, namely, the eyes and tentacles. In the hæmal region the integument may be modified and raised up into a fold at the edges, either in front or behind the anus. When so modified, it is called a mantle, Pallium. In front of the anus again, the branchiæ (t) project as processes of the hæmal region. Among the internal organs, the heart (u v) lies in front of the branchiæ in the hæmal regions, the nervous ganglia (x y z), of which there are three principal pairs, being arranged around the alimentary canal, which they encircle.
Such is the general type of the class Mollusca, of which, however, the variations are innumerable. They are all soft-skinned animals, without either articulated exterior or annular external skeleton. Their nervous system, being without cerebro-spinal axis, is entirely composed of ganglions, which are all reunited in the œsophagus without constituting in any case a lengthened median chain. Their digestive organs are complete—that is, they are provided with two apertures; their principal organs are symmetrical and according to a plan, usually curving, by which their bodies are divided into two parts.
The first series or subdivision, to which Milne Edwards has given the name of Molluscoïda, includes under that term the Bryozoaires, Ascidians, and Tunicata.