PHYLUM ECHINODERMATA 201 



usually more or less obscured by tube feet and spines, and 

 (2) by the presence of a complicated system of calcareous 

 ossicles which make up a chewing apparatus known as 

 " Aristotle's lantern." Sea-urchins are globular, discoidal, 

 or heart-shaped, and at first glance have little in common 

 with the starfish, but, if one could take a starfish and bend 

 the arms upward so that their ends came together and then 

 fuse the sides, he would have made a sea-urchin. The cal- 

 careous tests of sea-urchins make respiration difficult and 

 many species have therefore developed little brush-like 

 gills about the mouth. 



The cake-urchins and heart-urchins have the same general 

 plan of structure as the/ 'regular" sea-urchins, but have sec- 

 ondarily assumed bilateral symmetry. The cake-urchins, 

 or sand dollars (Fig. 82, F), have the anus at one side (in- 

 stead of at the pole opposite the mouth as in regular forms) , 

 and are very flat, so that they may move about on a soft 

 bottom without sinking in. The heart-urchins have the 

 mouth at one side and the anal opening at the other (B). 



CLASS 4. HOLOTHUROIDEA 



In the sea-cucumbers (Fig. 82, E) the calcareous exoskele- 

 ton, so characteristic of other echinoderms, is extremely 

 degenerate, being reduced to little spicules which are em- 

 bedded in the leathery skin. The chief axis is long, so 

 that the body is worm-like. There is a ring of tentacles 

 for feeding about the mouth. Respiration usually takes 

 place through the anus; water being pumped by muscular 

 action into two great respiratory trees, which branch out 

 among the internal organs and fill up a large part of the 

 space within the body. 



Sea-cucumbers show remarkable instances of autotomy, 

 or the casting off of parts of the body. One species, if re- 

 moved from the sand in which it is accustomed to burrow, 

 will break off pieces from its posterior end until only a twen- 

 tieth of the original body remains. Another species when 



