MOLLUSC A. 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 Meso- 

 podium {tn 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 haemal Avith the neural region, this forms 

 the Epipodiwn {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 Alesosoma. On the upper part of the sides 

 of the head are two pairs of organs, namely, the eyes and the tentacles. 

 In the hffimal 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 braiichiiz (/) project as processes of the hcemal region. Among 

 the internal organs, the heart (« v) lies in front of the branchiae in the 

 haemal 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 annulose external skeleton. Their 

 nervous system, being without cerebro-spinal axis, is entirely composed 

 of ganglions, which are all reunited in the oesophagus, 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. 



