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of Oncidimn or Peroma, These slugs are dirty-brown in colour, 
with wart'like tubercles on their back. The leaping fish 
Periophthalmus, so well-known through its huge prominent eyes, 
feeds according to Semper on these slugs. 
TOOTH -SHELLS {Scaphopoda), 
The Scaphopodd or Tooth-Shells have curved cylindrical 
shells, wide at the one end, tapering towards the other, with an 
aperture at either end. Not only in their shape» but also in 
their white colour they resemble a diminutive Elephant's tusk. 
At the wider opening are situated a rudimentary head which is 
provided with numerous long feelers, and a cylindrical foot 
which is adapted for burrowing in the sand. The Tooth-Shells 
are entirely marine, they occur in all seas, also around Great 
Britain, in shallow water to a depth of 2500 fathoms. They 
live buried in the sand, allowing only the thin posterior end to 
project, the opening of which serves at once as inhalent and 
exhalent aperture. Their radula proves a relationship with the 
Gastropods, whilst their rudimentary' head and other structures 
lead to the Bivalves. About 150 species of Tooth-Shells are 
known, most of which belong to the genus Dentalium. Speci- 
mens of it, dredged at Tanjong Pagar, Singapore, were presen- 
ted by Mr. H. W. Ford in 1902. They are about 2^ inches 
in length. 
BIVALVES (Lamembranchiata), 
The Bivalves are symmetrical, laterally compressed 
Molluscs and are, without exception, covered by a shell which 
consists of a right and a left valve. They differ from the other 
Molluscs by having no head and no radula, but a mantle is 
always, and a foot mostly present The mantle arises from the 
dorsal middle line of the body. It consists of a right and a 
left lobe which descend on either side, lining the valves, and 
may meet and fuse below to a greater or lesser degree, leaving, 
however, a slit for the passage of the foot. The edges of the 
mantle form behind two apertures, a lower inhalent and an 
upper exhalent one. The apertures may be simple gaps, or the 
margins of the mantle may be produced into long siphons. 
There are always sensory organs of some sort around the in- 
halent aperture. Bivalves live exclusively on minute organisms 
which enter by this latter aperture together with the water 
required for respiration, whilst the waste matter leaves by the 
