SHALLOW-WATER STARFISnES 23 



they are called paxillas. These are most typically formed in Astro- 

 pecten, Luidia, and allied genera, and are characteristic of the sub- 

 order Paxillosa.' 



Pedicellarije of one or more kinds are generally present, but are 

 entirely lacking or very rare in certain families, such as the 

 Solasteridae, Echinasteridae, Pterasteridse,' and in some genera of 

 other families. Their uses are imperfectly known. One use is to 

 keep the body free from dirt and parasites.' 



Another use, perhaps the most important, seems to be, according 

 to my own observations, to retain the discharged unfertilized ova 

 until fertilization takes place, thus preventing their dispersal and 

 loss. This, however, needs further investigations on living starfishes. 



These curious organs are not found in other classes of Echino- 

 derms, except in the Echinoidea. In the latter they are commonly 

 mounted on long stalks and generally have three valves, sometimes 

 two to four or even more. 



In the Asterioidea they are usually sessile or (Order Forcipulosa) 

 have very short pedicels, and they are most frequently bivalved. 



They are of several different types, which are commonly char- 

 acteristic of the orders and families, and often of the genera. 



Sometimes there may be bivalved, trivalved, four-valved, and 

 five-valved ones on a single specimen. (See below, under Der- 

 masterias imbricata, and pi. vi, figs. 4, 5 P, P'.). In some cases they 

 are not really valvular, but consist of several movable spinules, 

 arranged in convergent groups or opposed rows (pi. xxxiv, P. P.). 

 The sessile valvular forms are generally situated over a pore, pene- 

 trating a plate, for the passage of nerves, etc. (Pis. xlvii, xlviii.) 



The different kinds take special names. For further details of 

 the pedicellariae and their special names, see below, under the sev- 

 eral orders, and plates lxxv-lxxxv. 



In nearly all species there are numerous small, soft, tubular out- 

 growths from the body-wall, called papulae, serving for respiratory 

 purposes. They may be in large groups, especially between the 



' For various other special forms and their names, see below, under Phancr- 

 ozona and Valvulosa. 



' In the last two of these three families, and perhaps in the first, the mother 

 carries and protects the eggs and young until they are able to provide for 

 themselves. 



' The large bivalve pedicellariae of Hippasteria are very muscular. I have 

 lifted a large living starfish of this kind, weighing about a pound, entirely out 

 of water by a toothpick, seized by the jaws of a dorsal pedicellaria. 



