68 ANIMAL FORMS 
to the belief that the ancestors of some of these animals 
were segmented worms whose present condition has arisen 
possibly in accordance with their sluggish habits. This 
view is strengthened by the fact that in a very few species 
the larve are dis- 
tinctly segmented, 
but lose this char- 
acter in becoming 
adult. As before 
mentioned, the 
greater number of 
species live in bur- 
rows in the sand 
or crevices in the 
rocks, from which 
they reach out and 
gather in large 
quantities of sand. 
As these substances 
Fia. 42.—A gephyrean worm (Dendrostoma). Specimen pass down the in- 
on left opened to show &, kidney, m, muscle bands, testine the nutri- 
and 7.¢., nerve-cord. 
tive matters are di- 
gested and absorbed, while the indigestible matters are 
voided to the exterior. When large numbers are associated 
together they are doubtless important agents in modifying 
the character of the sea bottom, thus acting like the earth- 
worms and their relatives. 
71. The sea-mats (Polyzoa).—The sea-mats or Polyzoa 
constitute a very extensive group of animals common on 
the rocks and plants along the seashore, and frequently 
seen in similar situations in fresh-water streams. A few 
lead lives as solitary individuals, but in the greater number 
of species the original single animal branches many times, 
giving rise to extensive colonies. In some species these 
extend as low encrusting sheets over the objects on which 
they rest; while in others the branches extend into the 
