Dr. Williams on the Mechanism of Aquatic Respiration. 315 



to find a shelter from the radiation by concealing themselves 

 more completely among the rock-work or vegetation. Anthea 

 cereus is also very sensitive to considerable variations of tem- 

 perature, falling from its foot-hold to the bottom of the tank 

 apparently dead. 



Excess of heat and also strong sunlight are likewise to be as 

 carefully guarded against, and I may state as an evidence of this, 

 that on a particularly hot day during the summer of ISS-l, being 

 absent from home, the servant omitted to screen a small case 

 from the sun's rays during the hottest period of the day, and on 

 my return I found every creature dead. It contained an Anthea 

 cereus, Actinia dianthus, two specimens of Athanas nitescens, and 

 several others. 



Too much light has also the effect of rapidly propagating 

 several of the minute animalcules of a green colour, as the 

 Euglena and its congeners, which under this injfluence multiply 

 so rapidly as to render the whole water of a grass-green hue; 

 this will at times subside to the lower part of the tank as 

 evening approaches and disappear in the shingle bottom, but 

 immediatel}' the morning light shines strong upon the aquarium 

 it will rise like a thin green cloud and diffuse itself throughout 

 the whole of the water. Although this animalcular growth is 

 not unhealthy, yet it causes the aquarium to present a very 

 unsightly appearance, and prevents all observation on the habits 

 of the inmates. The want of light, I need hardly observe, 

 causes the rapid decay of the vegetation, and the products 

 arising from this change are highly poisonous to animal life, the 

 whole contents of the aquarium becoming of a black colour, aud 

 very soon of an offensive odour. 



Apothecaries' Hall, Sept. 11, 1855. 



XXIX. — On the Meclianism of Aquatic Respiration and on the 

 Structure of the Organs of Breathing in Invertebrate Animals. 

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

 Swansea Infirmary. 



[With a Plate.] 



[Continued from vol. xiv. p. 262.] 



Gasteropoda. 



It is now proposed to inquire into the structure of the organs 

 of breathing in that multitudinous group of mollusks which lies 

 above the Lamellibranchiata, constituting literally a great sub- 

 kingdom ; it will be impossible withiu the limits of this memoir 



