MOLLUSCA. 301 
muscular expansion or disc, 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 Afeso- 
podium (ms); and a posterior, the Aetapodium (mt). 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 hzemal with the neural region, this forms 
the Lpipodium (ep). 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 the tentacles. 
In the hzemal 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, Pal/izm. In front of the anus, again, 
the dranchie (¢) project as processes of the hemal region. Among 
the internal organs, the heart (w v) lies in front of the branchize in the 
heemal regions, the nervous ganglia (« 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 annulose external skeleton. Their 
nervous system, being without cerebro-spinal axis, is entirely composed 
of ganglions, which are all reunited in the cesophagus, without consti- 
tuting in any case a lengthened median chain. Their digestive system 
is complete—that is, it is 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 Molluscoida, includes under that term the Polyzoa and 
the Tunicata. 
