STAR-FISHES, SEA-URCHINS, ETC. 75 
The brittle-stars, or, as some choose to call them, 
feather-stars (Pl. 5, Fig. 5), are closely related to 
the star-fishes, differing mainly in that the arms 
radiate from a clearly-defined central disk, and, 
being more nearly solid, do not contain prolonga- 
tions of the stomach or of the ovaries in their in- 
teriors. The tube-feet pass out from the plates 
along the sides of the arms, instead of from the 
under surface as in the star-fishes proper. 
The brittle-stars are fairly active creatures, help- 
ing themselves rapidly over the sands by means of 
their long flexible arms. Their home is largely 
among the tangle and eel-grass, where their pro- 
tective coloring affords them a safe harbor. 
SEA-EGGS AND SAND-DOLLARS. 
Most visitors to the sea are acquainted with 
the appearance, at least, of the ‘sand-dollar’ (or 
‘sand-cake,’ as it is sometimes termed)—a thin 
shelly piece, from an inch and a half to three inches 
across, with a peculiar stellate ornamentation on 
one side, and irregular grooves and channels on the 
other. It immediately suggests a relationship with 
the star-fishes through its well-made star, although 
in a general way it appears to be a very different 
animal. Many people, indeed, scout the idea when 
informed that an animal inhabits this frail habita- 
tion, so unaccommodating does it appear; but an 
animal dwells within it nevertheless, and this ani- 
mal is truly a near relation to the star-fish. 
In its living condition, or when freshly removed 
from the water, the sand-dollar (Echinarachnius, PI. 
