48 ANIMAL FORMS 
are collected into two definite masses (Fig. 28, B), which 
constitute a simple brain on which the eyes are situated 
and from which bundles of nerve fibers pass to all parts of 
the body, the two extending backward being especially 
noticeable. As in the squirrel, these are distributed to the 
muscles and other organs to regulate their activity, while 
those distributed to the skin, especially in the forward 
part of the body, convey stimuli produced by touch. The 
branches connecting with the eyes enable the animal to 
distinguish light from darkness, but are probably too sim- 
ple to allow it to clearly distinguish objects of the outside 
world. The sense of smell and possibly that of taste are 
also present, but are relatively feeble. 
Some other characters of this class will be noted in the 
consideration of the two following classes. 
47. Parasitic flatworms (trematodes)—parasitism.—Men- 
tion has already been made of the associations of two ani- 
mals as “messmates” for mutual benefit, such as the Hy- 
dractinia growing on the surface of the shell inhabited by 
the hermit crab, to which it gives protection by means of 
its nettle-cells, while in turn being borne continually into 
regions abounding with food. More frequently, however, 
one animal derives benefit from another without making 
any compensation. For example, many species of flatworms 
live within the shells of certain snails and upon the bodies 
of sea-urchins and starfishes, where they gather in their 
food supply safe from the attacks of enemies. Such asso- 
ciations are probably without much if any inconvenience to 
the animal thus inhabited, and it also appears probable 
that the tenants are transients, using the mollusk or star- 
fish only as a temporary home. But from this condition of 
affairs it is only a short step to the parasitic habit, where 
the association becomes permanent and the occupant is 
provided with various structures which prevent its sepa- 
ration from its host. This latter kind of union occurs 
throughout the group of trematodes ; all are parasitic, and 
