96 



Mr. H. N. Moseley on the 



[June 15, 



Stylctster so closely resembling it, tbey are devoid of tentacles. In Allo- 

 pora they have twelve, in Errina four, in Acanihopora six, in Polypora 

 dickotoma four. In Polypoi^cty in which the tentacles of the alimentary 

 zooid were examined in the fresh condition, the tentacles were seen to be 

 clavate, the heads of the tentacles being somewhat elongate, not spherical 

 as in Millepora. I am as yet uncertain whether these tentacles are cla- 

 vate in the other genera. The point is difficult to determine in the ex- 

 tremely contracted condition of the organs in reagents. The tentacles of 

 these alimentary zooids are very short ; they are placed in a single whorl 

 at the base of the broadly conical hypostome. In Cryptolielia ?iR^'mth.Q 

 allied Stylaster the tentacleless alimentary zooids are flask-shaped, with 

 a conical projecting hypostome, as seen by Sars *. The rounded bottoms 

 of the zooids are blind and unconnected with the coenosarcal canals ; but 

 a series of canals radiate upwards from the sides of the flask to branch 

 and join the network above. The smaller zooids I have termed tentacu- 

 lar zooids, because, though invariably devoid of tentacles themselves, they 

 have the form of the simple elongate tentacles, and evidently must per- 

 form a tentacular function. In Polyjpora, Errina, and Acanihomra these 

 tentacular zooids are dispersed irregularly amongst the alimentary zooids | 

 in Cryptohella, Stylaster erubescent, and Allopora they are arranged in a 

 circlet around a centrally placed alimentary zooid m each so-called calicle 

 of the corallum. The bases of these zooids communicate by large vascu- 

 lar offsets with the general network of the coenosarc. The cavities of the 

 alimentary zooids are four-rayed in transverse section, and in Polypora 

 they divide at then' base into four large vascular trunks, which subdivide 

 to join the coenosarcal meshwork. The cavities of the tentacular zooids 

 are cncular in transverse section. Both kinds of zooids are provided with 

 strong circular and longitudinal muscles, which form wide conspicuous 

 bands beneath the ectoderm. The alimentary zooids are situate on the 

 summits of the styles of the coraUum, where these are present. In 

 Polypora, in the retracted condition of the zooids, the styles traverse 

 the axes of the zooids from below for at least two thirds of their length. 

 In Polypora, Errina, and Acantliopora the zooids of both kinds are re- 

 tracted within long sacs, the cavities of which communicate with the sur- 

 rounding network of the coenosarc by a series of radially disposed canals, 

 which canals in transverse sections of the zooids have at first sight exactly 

 the appearance of a system of mesenteries. In Cryptolielia and the Sty- 

 laster so closely resembling it the alimentary zooids, lying as they do 

 deep in the calicles, are probably never far protruded. The tentacular 

 zooids are partly retracted between the pseudo-septa, partly doubled 

 down within the calicles when the colony is in the retracted condition. In 

 the other Stylasters and in Allopora the conditions are much the same. 

 Two kinds of thread-cells are present, large and small : the large are of 

 the slightly curved cylindrical form, and emit a thread with an elongate 

 enlargement upon it near the sac, beset with a spiral of spines ; these 

 ^ Forh. Selsk. Chr. 1872, p. 115. 



