On the Structure of the Stylasteridae. 187 



Structure of the soft parts of tlie Stylasteriche. 



In all the Stylasteridae examined there is present an abundant 

 ccenosarc, made up, as in the Milleporidoe, of a network of anas- 

 tomosing canals, composed of an endoderm and ectoderm, and 

 ramifying in corresponding canals in the spongy trabecular cal- 

 careous coenenchym. In Fohjpora the meshes of the network are 

 comparatively close ; in all the other genera examined far more 

 widely open. In Cryptohelia and the iSti/laster from off the Mean- 

 gis Islands, in which the caUcles appear as swellings seated upon 

 slender connecting branches, bundles of larger canals traverse the 

 axes of these branches, and connect the zooid groups of the several 

 calicles with one another. A continuous layer of tissue, as far as 

 has yet been seen without cellular structure, but containing thread- 

 cells, covers the external surface of the caniosarc in all the genera. 

 In all the Stylasteridae there are two kinds of zooids, as in Mille- 

 pora; the larger and less numerous have mouths and a special 

 layer of digestive cells lining their body-cavity. The more numerous 

 smaller zooids have no mouths and no gastric cells. The alimentary 

 zooids are short and cylindrical ; the smaller or tentacular zooids 

 long and tapering. The alimentary zooids in Sti/laster eriobescens 

 have eight tentacles ; in Cryptohelia, and in the Stylaster so closely 

 resembling it, they are devoid of tentacles. In Allopora they have 

 twelve, in Errina four, in Acanthopora six, in Polypora dichotoma 

 four. In Polypora, 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. 1 am as yet uncertain whether these 

 tentacles are clavate in the other genera. The point is difficult to 

 determine in the extremely contracted condition of the organs in 

 reagents. The tentacles of these alimentaiy zooids are very short ; 

 they are placed in a single whorl at the base of the broadly conical 

 hypostome. In Cryptohelia and in the allied Stylaster the tentacle- 

 less 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 

 tentacular zooids, because, though invariably devoid of tentacles 

 themselves, they have the form of the simple elongate tentacles, and 

 evidently must perform a tentacular function. In Polypora, Erri- 

 na, and Acanthopjora these tentacular zooids are dispersed irregularly 

 amongst the alimentary zooids ; in Cryptohelia, Stylaster erubescens, 

 and Allopora they are arranged in a circlet around a centrally 

 placed alimentary zooid in each so-called calicle of the corallum. 

 The bases of these zooids communicate by large vascular offsets 

 with the general network of the coenosarc. The cavities of the ali- 

 mentary zooids are four-rayed in transverse section, and in Poly- 

 pora they divide at their base into four large vascular trunks, which 



* Forh. Selsk. Christ. 1872. p. ll.'i. 



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