INTRODUCTION. 31 



The nematocjjsts of the tentacules^ and of the general surface are the destroying 

 weapons ; their missiles paralyse and slay, whilst the spiral threads envelope and kill as 

 well. The spiral threads are observed in the corals with " serial" calices especially," and 

 the tentacules are not well developed in those species. The threads appear at the 

 calicular margin, and have openings through which they pass to and fro from the 

 visceral cavity. They are sometimes noticed in simple corals with well-developed 

 tentacules.^ 



Anything destroyed by the nematocysts of the tentacules, or killed by the spiral 

 threads, either falls on to the disc, or is passed on to the mouth directly and without the 

 agency of the cilia. The cilia are especially useful in passing small bodies towards the lips; 

 and these, when protruded, are moved in all directions seeking food. 



Once within range of the lips, the food is grasped by their sphincter and passed into 

 the stomach. 



The movement of the tentacules and of the lips is produced by the contraction of tlie 

 second or muscular tissue. All the tissues are very excitable, and contractions are readily 

 produced by irritation ; but the muscles act with a remarkable coordination, considering the 

 absence of the organs of vision and of all nervous structm-es. 



The stimulus of light acts very decidedly, so does that of heat, and direct contact 

 produces that series of changes which has been described by M. J. Haime. 



The stomach dissolves more or less of what goes into it, and passes the solution into 

 the visceral cavity through the pylorus, Avhilst the faeces are returned and rejected. No 

 acid reaction has been obtained from the stomachal membrane. Much water passes 

 through the stomach and into the visceral cavity. 



The visceral cavity receives the primarily assimilated food and the water which passes 

 through the stomach ; all this is brought in contact with the irrigatory system — with the 

 tissues lining the interloculi covering the septa, &c., with the mesenteric folds and the 

 tubular processes, as well as with the inferior surface of the disc and the bases of the 

 tentacules. Finally, this watery medium kept in agitation by the cilia of the visceral 

 membranes is now and then expelled through the tentacular orifices. A process of 

 absorption goes on, and the results of secondary assimilation appear to be the deposit 

 of the sclerenchyma and the nutrition of the soft tissues. 



Doubtless, the external tissues with their nematocysts have a power of retaining and 

 more or less absorbing nourishment without the process of digestion. 



The respiration of corals appears to be carried on by the tentacules, the membrane 

 lining the intermesenteric spaces — the ii'rigatory system, and by the general surface. 



1 Plate 11, figs. 1, 3, 5, G, 7, 8. ^ Plate II, % 17. » Plate II, fig. II.- 



