STRUCTURE OF A STARFISH. 207 



the ring round the anus, vessels are given off to the repro- 

 ductive organs and to the intestine ; from the ring round 

 the mouth, a radial vessel runs along the groove of each arm, 

 below the water-vessel and above the nerve. The fluid in 

 the blood-vessels contains brown cells, a few of which also 

 occur in the water-vascular system. 



Respiratory System. — From the dorsal surface and sides 

 of a starfish in a pool, you will see numerous transparent 

 processes hanging out into the water. They are the 

 simplest possible respiratory structures, contractile out- 

 growths of the skin, with cavities continuous with the 

 coelome, and are called "skin-gills." It is likely that the 

 brown cells of the body-cavity fluid act like rudimentary red 

 blood-corpuscles ; the water-vascular system may help in 

 aeration ; and the whole body is of course continually 

 washed with water. 



Excretory System. — It may be that the water- vascular 

 system has a slight excretory function ; it may also be that 

 excretion is somehow concerned in forming the carbonate 

 of lime skeleton, but facts are wanting. 



Reproductive System. — The sexes are separate, and they 

 are like one another, both externally and internally. The 

 organs develop periodically, and lie in pairs in each arm. 

 Each is branched like an elongated bunch of grapes, and 

 is surrounded by a blood sinus. Each has a separate 

 duct, which opens on a porous plate, between the bases 

 of the arms on the dorsal surface. The eggs are ferti- 

 lised in the water, and the free-swimming larva, which 

 will be described along with those of the other classes, is 

 known as a Bipinnaria or as a Brachiolaria. 



Other Starfishes. 



All Asteroidea are not of course exactly like the common type which 

 we have described ; thus Astropecten and most forms related to it 

 have blind food-canals ; Brisinga has long arms, arising abruptly from 

 a small disc as in Brittle-Stars, and has no ampullae ; Luidia has 

 three-bladed pedicellarise ; in most forms the genital ducts end on plates 

 with a single aperture, and so on. But as such differences are not 

 important for our present purpose, we shall simply notice a few of the 

 interesting forms. 



The commonest European forms are species of Asterias or Aster- 

 acanthion, Astropecten, Cribrella, Solaster, Goniaster. 



