66o 



ECI-IINODERMA— CLASS ECHINOIDEA. 



front margin, while the anus moves downwards to the hinder margin, and 

 eventually comes to lie on the under side of the test. These heart-urchins, 

 as they move along through the sand and mud, scoop it up into their mouths, 

 and pass it through the gut, extracting from it on its passage such nutriment 

 as the minute organisms in it can afford. Urchins of this type have short 

 delicate spines, as shown on the left side of Fig. 8. They move almost 

 entirely by stretching out their long tube-feet. It is, perhaps, the hair-like 

 spines that have caused the urchin here iigured to be popularly known as 

 Mermaid's head, Child's head urchin, and Hair sea-egg. 



The sea-urchins, like the star-fish, have pedicellarise. In the regular 

 urchin locomotion is chiefly effected by means of the spines, which are used 

 like crutches ; they can also be used like chop-sticks for prehension. The 

 spines also serve as organs of protection, but their efficacy varies much in 

 different forms. In Diadema setostim the spines reach a length of 10 in., 

 and are so fine that one is pricked by them before one can see them. A 

 few urchins, such as Asthenosoma sirens, have poison-glands attached to their 

 spines. On the other hand, the large spines of the Piper appear to be very 

 small protection against its natural enemies, fish and star-fish ; it is only the 

 small spines that have any defensive value, and they are placed for this purpose 

 near the main openings and organs of the body. Some sea-urchins cover 

 themselves with dead shells, sea-weed, and similar objects, which they hold 

 on by their tube-feet and so move about unobserved. Other sea-urchins do 

 not move about but live in holes in the rocks, which in some cases they can 

 be proved to have bored for themselves. The sea-urchins here figured have 

 a rigid test; but there are others in which the plates are not so closely joined 

 together, and the test is flexible. Fossil examples of such leathery urchins 

 have long been known from the Chalk, but were 

 not found in the living state before the dredging 

 expedition of the Porcupine. The regular sea- 

 urchins breathe by ten thin-walled extensions of 

 the body-cavity which are protruded around the 

 mouth. In the irregular urchins some of the 

 tube-feet are modified for respiration. 



Sea-urchins live on both animal and vegetable 

 food, and even on one another. They them- 

 selves are often eaten by fish, and some are 

 thought delicacies by man. The ovaries, when 

 in the spring they are full of eggs, are said to 

 equal the best fish-roe. 



HoLOTnuKoinBA (Sea-cucumbers). 

 A typical example of the sea-cucumber, 

 Cucnmaria planci, is represented in Fig. 9. 

 The body, as already said, is elongated, with 

 a mouth (M) at one end, and an anus (A) 

 at the other. Around the mouth is a fringe 

 of branched tentacles connected with the 

 water-vascular ring. Two of these tentacles, 

 those in the direction of the letter M, are 

 shorter than the others. Five rows of tube- 

 feet pass from the mouth to the anus, and under them lie radially-disposed 

 nerves and muscles. In no holothurian, however, does the radial arrange- 



Fi</. 9.— A Sea-cttcitmber 

 (Cucumaria planci). Natural size' 



