302 THE NATUEAL HISTOET EEVIEW. 



culated through the aquarium, and the waste carried off from below. 

 This habit of changing the water led accidentally to an observation of 

 some interest. A branch of coral which had for some time flourished 

 in captivity at other places on the coast remained in a contracted state 

 for a fortnight after being brought to Algiers. The water, renewed 

 with the greatest care, was obtained at the pier on the outside of the 

 harbour, but without producing any change. It was then suggested 

 by the sailor in attendance on the author that this water had been 

 too much aerated by continued breaking on the rocks, and a supply 

 from within the harbour would be more suitable. The experiment 

 was tried, and the polyps immediately expanded. This explanation, 

 adopted by M. Lacaze-Duthiers, may possibly be the correct one, 

 although opposed to general experience, and if we cannot at once 

 accept it as satisfactory it is because so many delicate conditions 

 besides that of aeration may be involved in such an experiment. 

 "We have called attention to this observation of the author's because 

 if confirmed, it may be of importance in the management of deep- 

 water animals, the study of which can be so materially assisted, as 

 in the present case, by the employment of a more or less well-regu- 

 lated aquarium. 



The structure of the polyps does not call for any lengthened 

 notice, agreeing as it does with that which has often come under 

 olDservation in Alcyonium digitatum — an easily accessible represen- 

 tative of the group characterized by the presence of eight pinnate 

 tentacula. "We must not, however, omit to mention that the author 

 points out an important distinction between the structure of the 

 thread-cells or nematocysts of the actinoid polyps, and of those of 

 Corallium. In the actinoid polyps the cell is simple and the thread 

 occupies the whole of the cavity ; but in Corallium, as well as in 

 the group generally to which it belongs, the thread is coiled up in 

 a small elongated cell placed in the middle of a larger one. We 

 could have wished that the author had pursued his inquiries further 

 into this subject, and thrown some light on the obscurity surrounding 

 the means by which the thread is extruded, and the origin of the 

 urticating properties that are so painfully evident on its insertion. 

 The subject, no doubt, is a difficult one, and requires the most careful 

 attention with high magnifying powers, and materials fresh from 

 the sea. With regard to the digestive organs of the coral polyp, 

 we are glad to observe that the author does not insist on the oeso- 

 phageal tube being considered the true stomach, as it is generally 



