THE SEA CUCUMBERS 3279. 



means of the tube feet, as a kind of concealment. Some sea urchins cover them- 

 selves all over in this way with bits of seaweed, shell, and small pebbles, and so 

 move about unobserved. Other sea urchins do not move from place to place, but 

 always stay in one spot, where they are generally found living in a hole. Some- 

 times the hole may have been there before the sea urchins; sometimes may have 

 been formed by the growth of calcareous algse around the sea urchin; but some- 

 times the urchin itself has bored the hole. This is accomplished not by any acid 

 secretion, for on the west coast of Africa an Echinometra has been found boring 

 into an augite lava, but by the continuous movement of the teeth and spines. 

 The common Strongylocentrotus is a well-known example of a boring sea urchin. 

 When the waves wash up against the urchin it sets its spines rigidly against the 

 sides of its hole and so holds fast. 



Although most of the sea urchins have a rigid test, yet there are some in which 

 the plates are only loosely joined together, so that the test is flexible. This is the 

 case in an Astropyga; but is still more pronounced in the leather urchin (Astheno- 

 soma), and other members of the family Echinothuridce , Respiration is effected in 

 the regular sea urchins by ten gills near the mouth. These are thin-walled ciliated 

 extensions of the body cavity protruding between the membrane round the mouth 

 and the plates of the test. In the irregular urchins some tube feet are modified for 

 respiration, becoming broad, flat, and somewhat lobed; the hinder end of the intes- 

 tine seems to be respiratory in function. Some sea urchins possess eyes. In a 

 Diadema there are five ovate pigment masses of a brilliant ultramarine blue, placed 

 at equal distances around the vent. There are certain other peculiar bodies sup- 

 posed to be sense organs of some kind, called sphaeridia, which are of microscopic 

 size, and in structure not unlike tiny spines. They lie near the mouth and on the 

 lower ambulacral plates, are often set in small holes, and are provided with special 

 nerves. Perhaps they test the water in which the sea urchin lives, and this may be 

 said to serve the sense of smell. Sea urchins are both animal and vegetable feeders, 

 and are even cannibals when opportunity offers. 



THE SEA CUCUMBERS Class Holothuroidea 



Sea cucumbers are, as we have seen, elongated and 

 worm-like creatures, with a mouth at one end and a vent 

 at the other. The skin is leathery, and contains a com- 

 paratively small amount of calcareous matter. Usually 

 this occurs in small spicules, which assume very definite 

 shapes, such as the anchors of Synapta, or the wheels of 

 Chiridota; but in such forms as Psolus the spicules in- tj. SHA p ED SEA CUCUMBER. 

 crease in size, so as to form a plated integument. There (Two-thirds natural size.) 

 may also often be a ring of calcareous plates round the 



gullet, five of which plates have the same relation to the radial water vessels as the 

 auricles around the jaws of a sea urchin, and they likewise serve for the attachment 

 of muscles. In such a common form as Cucumaria planci there are five rows of tube 



