CHARACTERS OF INVERTEBRATE ANIMALS 



Fig. 281. Three-jawed Pedicellariae of Sea- 

 Urchins ; greatly enlarged 



ambulacral areas, and the regions between them into the five 

 interambulacral areas. 



There yet remain one or two other points to be noticed in 

 the sea-urchin test. ' The plates at the upper pole, in the neigh- 

 bourhood of the intestinal aperture, 

 are somewhat specialized, and consti- 

 tute an apical disc, the most important 

 elements in which are ten plates, of 

 which the five smaller ones are situated 

 radially, and therefore correspond with 

 the upper ends of the ambulacral areas. 

 Each of them is called an " ocular ", 

 because it bears an eye -spot. The 

 five interradial plates are somewhat 

 larger, and one of them is modified 

 into the madreporite, which has the 

 same function as in an ordinary star- 

 fish (see p. 452). Like that animal, 

 too, the sea-urchin possesses those 

 modified jaw-spines to which the name 

 of pedicellarice has been given, but in this case the jaws are three, 

 and not two in number (fig. 281). Some possess poison-glands 

 (fig. 281, right). 



The digestive organs consist of gullet and intestine, the latter 

 taking a spiral course, while the former traverses a complicated 

 chewing apparatus fancifully called " Aristotle's lantern", and made 

 up of the five jaws, together with many other hard parts. By means 

 of special muscles the jaws can alternately be brought together 

 and separated. As in the other groups of Echinoderms so far 

 dealt with, there is a blood system of rather obscure nature, and 

 the rest of the circulatory organs consist of a particularly spacious 

 body -cavity in which the various internal organs are placed. 

 Breathing is effected by the tube-feet, and also by ten branching 

 gills placed in the neighbourhood of the mouth, and resembling 

 the structures which project between the interstices of the skeleton 

 in a star-fish (p. 453). This function is also possibly assisted by 

 a narrow tube (siphon) running parallel to the gut and opening 

 into it at either end. The water-vascular system is arranged on 

 the same general plan as in a star-fish (p. 453), consisting of a ring 

 round the gullet connected with the madreporite by a stone-canal, 



