STAR-FISHES, SEA-URCHINS, ETC. 75 



The brittle-stars, or, as some choose to call them, 

 feather-stars (PI. 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-DOLLABS. 



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. 



