VII MOLLUSGA— OUTER ORGANISATION 29 



A. Plaeophora or Polyplaeophora (Chitonidse). 



The body of the Plaeophora is bilaterally symmetrical, and dorso- 

 ventrally flattened ; viewed from the dorsal or ventral surface its 

 shape is that of a long oval. On the ventral side there is a large 

 muscular I'oot vi^ith a flat sole, the outline of which runs very nearly 

 parallel with that of the body. In front of the foot, and also on the 

 ventral side, there is a distinct snout which carries the mouth in the 

 middle of its ventral surface. There are no eyes or tentacles on the 

 head. Between the mantle, which forms the outer edge of the body, 

 and the body and head it covers, there is a deep groove, in the base of 

 which lie numerous lancet-shaped gills, arranged in a single row on each 

 side. These two rows of gills sometimes approach each other so nearly 

 both anteriorly and posteriorly that there is an almost complete circle 

 of gills around the foot, or else they are more or less shortened, and 

 are in some forms so reduced as only to occupy the posterior third 

 of the branchial furrow. The anus lies posteriorly in the median 

 line, ventrally, immediately behind the foot. The two apertures 

 of the nephridial ducts lie in the branchial furrow on each side, and 

 slightly in front of the anus. The two genital apertures lie imme- 

 diately in front of the nephridial apertures, also in the branchial 

 furrow. 



The median dorsal region is covered by eight consecutive imbri- 

 cating calcareous plates. The peripheral dorsal region, between the 

 edge of the body and these shell plates, carries calcareous spicules, 

 granules, etc. The corresponding peripheral region on the ventral 

 side forms one of the boundaries of the branchial groove, and may be 

 considered as the mantle. 



B. Aplaeophora, Solenogastres. 



The body is here bilaterally symmetrical and vermiform ; in section 

 it is round, and is sometimes long and thin, at others short and thick. 

 The large oral aperture lies in the form of a longitudinal slit on the 

 ventral surface of the anterior end of the body. The cloacal aperture 

 — or common opening for the intestinal canal and the urogenital 

 organs — lies ventrally at the posterior end of the body. A narrow 

 median ventral groove runs forward from the cloacal aperture 

 and terminates anteriorly near the mouth. In the base of this pedal 

 groove rises a ciliated ridge or fold which runs along its whole 

 length; this ridge, in cross section, is triangular, and represents 

 the reduced foot. In the Chcetoderma both foot and pedal groove 

 are wanting. The Solenogastres have no distinct compact shell ; 

 its place is taken by calcareous spicules embedded in the integu- 

 ment. 



