OF COENOPSAMMIA FROM LIFU. 363 



of nematocysts. 1, 2 and 3, Primary, secondary and tertiary mesenteries, consisting of two 

 layers of endoderm with the structureless membrane between. The retractor muscles are 

 indicated as fine lines at right angles to the mesenteries, -m.f. mesenterial filaments. 



Sections. 



A. Above the theca. The primary septa are represented although they do not, except 

 in young corallites, project above the theca. 



B. About half-way between the top of the stomodoeum and the top of the polyp. The 

 theca is complete and divides the coelenteron into extra- and intrathecal portions, connected 

 together by numerous perforating canals, commonly running as represented. The extrathecal 

 portions of the coelenteron are divided into canals, which correspond in number and position 

 to the spaces between the mesenteries. The costae and secondary septum are well marked and 

 the tertiary retractor muscles have appeared. 



C. Through the mouths of the retracted tertiary tentacles, which lie externally to those 

 of lower orders. The retractor muscles of the secondarj' mesenteries have appeared and the 

 tertiary septa are indicated. (The perforating canals of the corallum and the peritheca are 

 not represented in this and subsequent sections.) 



D. Through the mouths of the retracted primary and secondary tentacles. The polyp 

 is cut across in two places owing to the somewhat raised lip round the stomodoeum. 



E. Through the lower half of the stomodoeum. The lower free ends of the retracted 

 primary and tertiary tentacles are well marked. The tertiary mesenteries are free with 

 distinct filaments ; in the left pair ova are represented, covered by the much thickened 

 nutritive endoderm. 



F. Towards the base of the polyp. The right half of the section is rather higher than 

 the left. In it the tertiary mesenteries are much narrower with smaller filaments and 

 without ova, which have now appeared in the secondary mesenteries. In the left half the 

 tertiary mesenteries have lost their filaments and muscles and will shortly disappear; the 

 secondary mesenteries are narrower and have lost their muscles, while in the primary 

 mesenteries ova have appeared. 



The contracted polyp. In the completely contracted condition the opening of the 

 stomodoeum is situated about Vh mm. above the top of the columella. The peripheral 

 part of the mouth-disc is drawn inwards and downwards, causing a marked depression 

 round the stomodoeal opening. This condition is brought about by the presence of 

 a strong circular sphincter muscle — Rotteken's muscle of the Actiniaria — (Fig. II. m.s.) 

 together with that of sti'ong longitudinal muscles on the mesenteries, spreading out 

 under the oral disc and within the tentacles. The presence of a circular muscle in 

 the Madreporaria was first suggested by Moselcy (21) in Sphetiotrochus and subseciuently 

 described by Fowler in the same genus (9) and also in Dwncania (11). But, whereas 

 in these genera the sphincter opening leads into a cavity in which the tentacles 

 lie, the condition differs in Coenopsammia owing to the complete introversion of the 

 tentacles in a more irregular manner but similar to that of Seriatopora (9). In fact 



