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 



