Dr. T. Williams on the Sand-canal in the Starfishes. 55 



VI. — On the Uses of the Sand-canal in the Starfishes. By 

 Thomas Williams, M.D., F.L.S., Physician to the Swansea 

 Infirmary. 



To the Editors of the Annals of Natural History. 



Gentlemen, Swansea, Dec. 22, 1856. 



I HAVE recently accidentally fallen upon a very simple expe- 

 riment, which brings directly under the eye at least one of the 

 functions of the " Sand-canal " of the Asteriadse. Let a spe- 

 cimen of the familiar Asterias ruhens in the fresh state be allowed 

 to creep gradually from the bottom up the sides of a dish, and 

 then pour into the dish very slowly fresh salt water ; proceed 

 until the level of the water accurately corresponds with the infe- 

 rior (as the specimen is placed) edge of the " eye," or the madre- 

 poriform tubercle, which is the external outlet of the sand- 

 canal. Now fix the attention upon the smooth calm surface of 

 the water. In a moment or two a powerful current will be 

 observed. It commences at the eye or madreporiform tubercle, 

 and travels in enlarging waves towards the sides of the vessel. 

 In a minute or two more the phsenomenon occurs again, and 

 again and again at rhythmic periods. Now fix the eye intently 

 upon the surface of the water in the vicinity of the " tubercle," 

 in order to discover whether, during the intervals between these 

 rhythmic currents, a contrary movement of the water takes place, 

 that is, whether an ingoing alternates with the outgoing current. 

 Try the experiment under every possible variation of circum- 

 stances, and you will convince yourself that it does not take jilace. 

 The function of the eye or madreporiform tubercle then is to 

 discharge externally from time to time the contents of the am- 

 bulacral or water-vascular system of Asterias. 



If analogy may guide the reason, the outlet of the sand-canal 

 of Asterias should be viewed as the exact equivalent of the 

 "fissures " in the neck of the Nemertine Annelids, which, as I 

 have recently proved (Dr. Carpenter first conjectured the fact), 

 open in a peculiar manner into the vascular system. I could not 

 rest satisfied until I had tried the question, whether the water 

 entered into or issued from these "fissures." I placed an indi- 

 vidual of the species — Polia (P. quadrioculata) — in a flat glass- 

 cell under the microscope, and watched with very fixed gaze the 

 neighbourhood of the fissures. I ascertained with perfect certainty 

 that from time to time an emission of fluid takes place at these 

 points ; but in no instance could I satisfy myself that any inspi- 

 ration of fluid from without succeeded to the expiration. From 

 these and other reasons I have ventured to deduce the following 

 conclusions : — 



