464 TEXT-BOOK OF ZOOLOGY 



tendril-like climbing feet (cirri) creep about on seaweeds, etc. Their 

 microscopic food (see p. 438, Section d) is swept by means of cilia, which 

 cover the floor of the brachial grooves, towards the mouth, which is 

 directed upwards. 



CLASS IV.: SEA-URCHINS (ECHNOIDEA). 



WITH this division, which is rich in varied forms, we shall make our- 

 selves acquainted by briefly considering one species, viz., the Common 

 Sea-Urchin (Echinus esculentus). This animal lives in the seas of 

 Northern Europe, and has but little resemblance to the star-fishes. By 

 imagining, however, the arms of a star-fish curved over its back in such 

 a manner that their margins touch each other, and, further, the cal- 

 careous plates fused into a solid shell, or " test," we arrive at the apple- 

 shaped body of the sea-urchin, with the mouth below, the anus above, 

 and between the two, five rows of pedicels. The shell, or " test," bristles 

 all over with sharp movable spines, which point in all directions, and 

 serve both as excellent weapons of defence (compare with hedgehog ; 

 name) and also as stilts, on which the animal strides along. The 

 principal organs of locomotion, however, are the pedicels. By their aid 

 the sea-urchin is even able to climb with dexterity and to ascend those 

 marine plants which form its special food. For cutting off and dividing 

 its food the animal is provided with five sharp teeth, which project 

 slightly from the oral opening, and the muscles of which are attached to 

 a framework of calcareous triangular pieces (alveoli). 



CLASS V.: SEA-CUCUMBERS (HOLOTHUROIDEA). 



IMAGINE a sea-urchin much drawn out longitudinally, so that a roller- 

 shaped body is formed, with the mouth at one end and the anus at the 

 other, and we have an* animal of the shape displayed by the sea- 

 cucumbers. The skin in these animals, however, is leathery, and con- 

 tains only small isolated calcareous bodies, whilst the mouth is sur- 

 rounded by a crown of, frequently branched, tentacles. Several species 

 found on the coasts of the East Indian and Australian seas are in large 

 quantities prepared into trepang, which is a favourite delicacy in China. 



