INTRODUCTION. 31 
The xematocysts 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 the 
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 structures. 
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, whilst 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 
ling the intermesenteric spaces—the irrigatory system, and by the general surface. 
ee Plate 1], fies..1, 3,5, 6, 7, 8: 2 Plate II, fig. 17. 3 Plate II, fig. 11: 
