CILTATIOSf OF ASTER-IDS. 13 



its products, or simply the great distention, having a markedly 

 injurious effect. 



9. If the oral surface of a Porania be sharply irritated, the 

 spines at the interradial angles of the mouth will close in and 

 by interdigitating with each other will cover up the mouth- 

 opening completely. In the same way the whole or any part of 

 an ambulacra! groove (see PI. I. fig. 1 Amb. Gr.') can be entirely 

 shut in by the spines on opposite sides of the groove. We seem 

 to have here a ready means of protecting the mouth from exposure 

 to streams of inacceptable or injurious particles. 



10. It may be said that all the details given above are to be 

 explained in terms of endodermal respiration. Probably this kind 

 of respiration occurs in our species, as, indeed, it does in other 

 aquatic animals (7). Certainly in captivity Porania exhibits 

 remarkable power of healing after injuries to the body- wall. But 

 were internal respiration the only or the principal function, one 

 would expect this function to be cared for in other starfishes, and 

 particularly in such a starfish as Asterias rubens, which feeds in 

 the ordinary way far more greedily and must accordingly exhibit 

 far greater tissue change than Porania. Yet inward currents 

 of water through the mouth cannot be observed in A. rubens, 

 nor does rhythmic expulsion of water through the anus occur. 

 However, for final evidence on the relation of ciliary action 

 to nutrition we must have recourse to observations on the 

 behaviour of Porania and other starfishes under circumstances 

 which preclude them from obtaining food by any other means 

 than the action of ciliary currents. Accordingly the following 

 and similar experiments were instituted. 



A starfish after being carefully weighed was placed in a bell- 

 jar, the wide end of which was covered by hair-cloth of fine 

 mesh, while the narrow end was connected to a siphoning tube. 

 Next, the bell-jar was immersed in one of a series of tanks with 

 continuous sea- water circulation, and outward siphon action was 

 started by means of the tube from the bell-jar into another tank 

 set lower in the series. Constant change of water within the 

 bell-jar was thereby assured, and at the same time the entrance 

 into the bell-jar of all objects of any size was effectually pre- 

 vented. At intervals the starfish was taken out and weighed 

 and the interior of the bell-jar cleaned. The first specimen of 

 Porania was put in on Feb. 28, 1914. At the end of four and 

 a half months, the mean of several weighings clone within the 

 last week of this period was practically the same as the mean of 

 the weighings done in the first week of the period. Thus 

 nothing at all was lost between the end of February and the 

 middle of July, that is, during the period when microscopic food- 

 particles are most abundant in the tanks. Since July there has 

 been some loss of weight, but the specimen is still healthy 

 (October 18). Several other Porania similarly treated have' 

 remained healthy for almost as long a period, and the smallest 



