XVI SKELETON PAPULAE-— WATER-VASCULAR SYSTEM 457 



short and blunt. The terms " monacanthid " and " diplacanthid " 

 are used to express the occurrence of one or two rows of spines 

 respectively on each adambulacral plate. 



In the Zoroasteridae the adambulacral plates are curved, and 

 are alternately convex and concave towards the ambulacral 

 groove, so that this groove presents a wavy outline. 



In the description of Asterias it was pointed out that the 

 first adambulacral plates in adjacent radii are closely approxi- 

 mated to one another, and bear spines which can to some extent 

 form a trellis-work over the mouth. In very many species not 

 only is this the case, but the plates themselves project inwards 

 over the mouth so as to form prominent "mouth-angles." This 

 is not the case in the Asteriidae or the allied families. 



Papulae. — In Asteriidae and many allied families these 

 organs are found both on the upper and under surface of the 

 disc, but in another large group consisting of Astropectinidae, 

 Pentacerotidae, and allied families, papulae are only borne on 

 the dorsal surface, and, in some cases, are restricted to a few 

 groups at the base of the arms. In most Asteroidea the papulae 

 are arranged singly, that is to say, each occupies one of the 

 interspaces between the plates of the skeleton, but in Asterias 

 and some other genera they are arranged in tufts of two or 

 three. 



Water -vascular System. — In its general structure this 

 system of organs is very constant, the two most important 

 variations being found, one, in Asteriidae and a few allied 

 families, and the other, in the Astropectinidae and the families 

 allied to them. 



The first of the variations alluded to concerns the number of 

 the tube-feet in a radius. In Asterias and its allies these are 

 so numerous that there is not room for them one behind the 

 other, but they follow one another in a zigzag line, the trans- 

 verse canals connecting them with the radial canals being 

 alternately longer and shorter. In this way the appearance of 

 four rows of tube-feet is produced, and the advantage of this 

 increase in number can be recognised by any one who has 

 compared the quick movements of Asterias and the slow ones 

 of a Crihrella, for instance. 



The second important variation referred to is the complete 

 loss of the sucker of the tube-foot, and, concomitantly, the loss 



