12 DR. J. F. GEMMILL ON THE 



large, and are present uniformly in all the interradii. They 

 have distinctly muscular walls which contract and expand 

 rhythmically at short intervals, sometimes with such activity as 

 to suggest the systole of the auricular portion of a heart. As 

 observed in living preparations, the contractions do not usually 

 affect all the caeca, or indeed the whole of a single caecum, at one 

 time, but probably in nature minor contractions of the caeca or of 

 parts of them are continually occurring, serving to cause changing 

 of the contained fluid. It will be remembered from pp. 7-8 

 that there is no ciliary provision for outgoing currents from the 

 rectal caeca. Presumably, it is the more or less simultaneous 

 contraction of the whole set that produces the periodical ex- 

 pulsion of water through the anus, which was referred to under 4 

 (p. 11). Turgidity may provide the stimulus to this act. At 

 any rate, simple pressure by the fingers on the aboral aspect of a 

 distended Porania will often, after the lapse of a few seconds, 

 induce a perfectly typical expulsion of fluid by the anus. The 

 body-wall in Porania is unusually thick and elastic, and when 

 the anus closes these properties may be of tise in causing negative 

 pressure within the different parts of the digestive cavity, and 

 thereby aiding the parts to become filled again with fluid entering 

 by the mouth and loaded with particles collected by the actinal 

 ciliation. The interradial or rectal caeca of Asterias rubens and 

 of the majority of starfishes are much smaller and less uniform 

 than those of Porania. However, alike in Asterias rubens, 

 A. glacialis, Solaster papposus *, and >S'. encleca, these caeca show 

 contractility, and probably have to do with the passage of material 

 along the food-canal and with the evacuation of faeces. 



8. After specimens have been deprived of solid food for a time, 

 the addition to the aquarium of finely-divided nutritive material, 

 e. g., debris from the ovary of a sea-urchin, spermatozoa, etc., 

 is almost invariably followed in the course of a few hours by 

 an increase in the weight of the specimen. This increase is 

 lost a day or two after replacement in clean water, and is, I 

 think, to be explained by the taking for the time being of 

 extra fluid into the digestive cavity along with the suspended 

 food-particles. Specimens kept in a glass vessel, through the wall 

 of which their oral surfaces can be observed with the help of a 

 microscope, will often be found to respond to agitation of the 

 water near them by opening the mouth, partially protruding the 

 pharynx, and actively swallowing food-particles. One must 

 exercise much caution in attempting to feed Porania with sperm. 

 Individuals, if left too long (24 hours) in a mixture showing only 

 slight milkiness, may distend themselves with water to an ex- 

 treme degree, sometimes reaching more than twice their former 

 weight. This is usually followed, in my experience, by loss of 

 vitality leading to death within a week or ten days, the sperm or 



* See footnote on p. 15. 



