SEA URCHINS 169 



The habits of sea urchins are interesting, and may be watched 

 in the aquarium, where the movements of the spines and of the 

 tube-feet may be seen perfectly. Some species are very inactive, 

 living in holes and crevices, or under stones, and seldom move from 

 their hiding-places, while others travel considerable distances. 

 The former have generally no eyes, and, instead of seeking their 

 food, simply depend for their subsistence on the material carried 

 to them by the movements of the water ; while the latter possess 

 visual organs similar to those observed in certain starfishes. Some 

 species also protect themselves from their enemies when in the 

 open by covering their bodies with sand, small stones, shells, or 

 weeds, and thus so perfectly imitate their surroundings that they 

 are not easily detected. The feet that are used for purposes of 

 locomotion terminate in suckers resembling those of the common 

 five-fingered starfish, and have considerable clinging power, but 

 some have either very imperfectly developed suckers or none at all, 

 and are probably used as feelers only. 



Sea urchins, like their allies the starfishes, generally inhabit 

 deep water beyond low-water mark, where they often exist in 

 enormous numbers, feeding on both animal and vegetable substances; 

 but some species are often to be met with between the tide-marks, 

 where they may be seen under stones, and frequently half hidden 

 in mud. The globular species occur principally on rocky coasts, 

 but the heart urchins are more commonly dredged from banks of 

 sand or mud that are always submerged. 



The life-history of urchins closely resembles that of starfishes, 

 for the young are free-swimming creatures of an easel-like form, and 

 during this early larval existence their bodies are supported by a 

 calcareous skeleton. 



We will conclude our short account of the British echinoderms 

 with a description of the peculiar Sea Cucumbers, which belong to 

 the division Holothuroidea. These creatures are so unlike star- 

 fishes and urchins in general appearance that the uninitiated would 

 hardly regard them as close relatives. The body is, as the popular 

 name implies, cucumber-shaped, with the mouth at one end, and 

 the general aspect is wormlike. There is, however, a radiate 

 symmetry a five-fold arrangement of parts, though not so regular 

 as in most echinoderms. Running lengthwise along the body are 

 five rows of tube-feet, but only two of these are well developed 

 and terminate in functional suckers; and, as might be expected) 

 the animal crawls with these two rows beneath it. The feet are 



