ECHINODEKMATA. 7 



the ambulacral plate corresponding to each pair of pores : and 

 this arrangement would appear to hold good for all the EM- 

 noidea except the Clypeasteridm, where the pores of the loco- 

 motive feet are grouped together on a single plate.' (Huxley.) 



Each row of ambulacral plates carries two rows of alternating 

 conical tubercles, and in addition there are inner tubercles of 

 various sizes scattered between the larger ones : these tubercles 

 carry the spines, which beset the external surface of the skeleton. 



The inter -ambulacral plates, though similar in shape, and 

 carrying tubercles, are larger in size than the ambulacral, and 

 imperforate. If an ambulacrum be traced upwards towards 

 the apical pole of the shell, it will be found to abut upon 

 one of the five smaller ossicles which form the outer row of 

 circum-anal plates. Each of these angular plates is perforated, 

 and supports an eye- spot. Hence they are called 'ocular plates.' 

 The apical extremities of the inter-ambulacra are similarly 

 related to the larger plates which form the inner series of cir- 

 cum-anal ossicles, and alternate with the ocular plates. These 

 plates, of the inner row, are likewise perforated with a small 

 aperture for the passage outwards of the generative products. 

 Hence they have been termed ' genital plates.' One of these 

 is much larger than any of the others, and is marked by a 

 porous elevation, the madreporic tubercle : which internally is 

 in relation with the sand canal. 



The small area within the ocular and genital plates is 

 occupied by small calcified tubercles and granules, within 

 which the anus is situated. The growth of the shell in the 

 Ecliinidc& is effected in two ways : partly by addition to the 

 circumference of existing plates, and partly by the interposition 

 of new ambulacral and inter-ambulacral plates at the apical end 

 of each series. The tubercles which cover the plates terminate 

 in rounded, ball -like summits, either simple, or marked by a 

 small pit. The spines articulating with these tubercles have a 

 cup-shaped depression at their attached extremity, which re- 

 ceives the rounded end of the tubercle in such a manner as to 

 form a ball and socket joint. The spine is attached to the 

 tubercle partly by a ligament which passes from the bottom of 

 its articular surface to the pit in the tubercle : and partly by 

 the muscular fibres which connect the neck of the spine with 



