78 
THE TASMANIAN NATURALIST. 
The next phylum is probably the largest of all, including as it does 
all the crabs, crayfishes, lobsters and other relatives, insects, millipedes, 
centipedes, spiders, scorpions, ticks, &c. 
The whole phylum is included in the name Arthropoda t and is 
divided into classes as follows :— 
Crustacea : lobsters, crayfishes, crabs, shrimps, &c. 
Myriapoda (many footed animals): centipedes and millipedes. 
Insect a : insects. 
Arachitida : spiders, scorpions, ticks. 
Of all these classes two have already been described in the society’s 
journals ; the Crustacea , by Mr. Geoffrey Smith, and the Insecta , by 
Mr. A. M. Lea. Of the other two classes there is nothing much to be 
said ; they are not as a rule sea animals, and the only time any of them 
are found in the ocean is when they may be blown there by wind or 
reach it by some other mishap. We will, therefore, leave these and pass 
to a large and important group of animals, a great proportion of the 
members of which are marine, the Mollusca or soft bodied animals. 
In the phylum Mollusca are included quite a number of different 
animals from the fresh water mussel and sea mussel, through the various 
kinds of snails up to the highly developed cuttlefish and nautilus. 
A simple but thorough review of the mollusca has already been 
given in this journal, by Mr. W. L. May, ar.d in view of that article we 
need here only attempt a very short description. 
Besides the shell a characteristic structure in the Mollusca is the 
arrangement known as the 1 foot,’ and it is on the development of this 
that the main division of the phylum Mollusca are founded. For 
example, we have first of all those members in which the foot consists 
of a wedge-shaped body which may or may not be pushed out of the 
shell (mussel, scallop, and others); secondly, there are those animals 
whose foot consists of a fiat expanded portion of the body on which 
they crawl (snails, slugs, tritons, &c.); thirdly, we find a number of animals 
who have the foot extended forward past the head of the animal and 
divided up into a number of tentacles (cuttlefish, octopus, nautilus, &c.) 
It is in the first of these groups, the class Pelecypoda, including 
mussels and scallops that the foot is least developed. It is used mostly 
for moving through mud and sand. In the scallops, however, movement 
is caused by flapping the two shells. In oysters, which are incapable of 
movement, the foot is absent. To see the foot in a scallop, separate the 
two shells, and the foot is visible as a plough shaped structure hanging 
from the centre of the body. 
As regards the shell the above three divisions vary greatly. In the 
Pelccypoda the shell is in two parts or valves, in the snails and allies 
(Gastropoda) the shell is single and usually consists of a cone which is 
spirally twisted. In the Cephalopoda (cuttlefish, octopus) the shell is 
usually quite or nearly enclosed in the body, and is merely the remnant 
