REPOET ON COEALS — HELIOPORID^E. 115 



the polyps of the Heliopora. Hence, possibly, the observed lack of propagation of applied 

 stimulus. In their later work 1 they write as follows : — "M. de Blainville's observations 

 on the animal of Heliopora ccerulea led him to remove it from the genus Pocillopora, in 

 which it had before been placed, and to form the genus Heliopora for it, because the 

 animals of Pocillopora have never more or less than twelve tentacles. Heliopora has 

 either fifteen or sixteen short, broad, triangular, pointed tentacles forming a disc around 

 the mouth. The animals were made out with difficulty with a powerful lens." Eight 

 compound tentacles appear to have been mistaken by the observers for sixteen simple 

 ones. 



Stomach. — The stomach of Heliopora is closely similar to that of other Alcyonarians. 

 As seen in the contracted condition its walls are horizontally plicate. In transverse 

 sections, as Plate I. fig. 3, the layers composing its walls are well seen. There is the 

 usual covering of the endoderm ; but in the mesoderm, within the layer of homogeneous 

 connective tissue, a second narrow zone (PI. I. fig. 3, B) can be detected which is 

 probably muscular. The inner ectodermal lining of the stomach is continuous with that 

 of the tentacles, but ciliated. 



Mesenteries and Muscles. — Eight mesenteries completely divide the upper part of the 

 cavity of the polyp into eight radially-disposed chambers. The mesenteries consist of a 

 median plate of homogeneous connective tissue, which is directly continuous with the similar 

 layer of the lining membrane of the calicular cavity, and also with that surrounding 

 the stomach, and of an investment of endodermal cells covering the median plate on both 

 sides, excepting where the retractor muscles intervene between the two.. The retractor 

 muscles form the lower borders of the mesenteries ; they consist of long stout fibres 

 which, lying on the surface of the mesenteries, take origin from the lower part of the 

 sides of 'the polyp-cavity, and reach sometimes as far clown as the margin of the tabula. 

 They curve thence inwards and upwards, becoming gradually more concentrated as they 

 ascend, and are inserted round the mouth and region just below it, in the intervals 

 between the bases of the tentacles. 



The muscles have in position, with regard to the plates of the mesenteries, the same 

 arrangement which Kolliker 2 has described as existing in the Pennatulidas, 3 and which 

 has also been found in the genus Umbellula by Lindahl and figured by him. 4 



The arrangement of the muscles is seen in Plate I. fig. 3, where E M, R M are 

 the muscles. At opposite ends of the long axis of the stomach the muscles are on 



1 Quoy et Gaimard, Voyage dela Corvette TAstrolabe, Paris, 1832, t. iv., Zoophytes, pp. 252, 253. 



- Prof. Kolliker, Anatomisch-systematische Beschreibung tier Alcyonarien, Abh. dersensk. naturf. Gesell, Frankfurt. 

 1870. 



s " Om Pennatulidslagtet Umbellula," Kongl. Vet. Akad. inleirmad den 10. Feb. 1874 : Stockholm, tab i. fig. 8. 



4 Professor Schneider and M. Rotteken must certainly have been mistaken in their conclusions concerning 

 the arrangement of the muscles with regard to the mesenteries in Alcyonaria, if the figure given in the Ann. 

 and Mag. Nat. Hist., 1871, vol. vii. p. 437, as representing them be correct. 



