On the. Structure of the Stylasteridffi. 187 



iStructure of tlit; soft purts of tht Styldsteridce. 



In nil the Stylasterida) examined there is present an abundant 

 cocnosiirc, made up, as in the Milleporid;c, ol" a network of anas- 

 tomosing canals, composed of an endodcrm and ectoderm, and 

 ramifying in corresponding canals in the spongy trabecular cal- 

 careous cumenchym. in J'uli/jiora the meshes of the network are 

 comparatively clo>e ; in all the other genera examined far more 

 widely open. In Crifptoh-lia and the ^7y?f(.s(e/- from off the Mean- 

 gis Islands, in which the calicles 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 ca'uosarc in all the genera. 

 In all the Stylast^^ridte 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 Styhister eruhi'scens 

 have eight tentacles ; in Cri/ptohcUa, and in the >'Sfi/laster so closely 

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

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

 four. In Fohipora, 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 Millrpora. i am as yet uncertain whether these 

 tentacles are clavate in the other genera. The j)oint is ditlicult to 

 determine in the extremely 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 CnjptoheUa and in the allied Sti/Iaster the tentacle- 

 less alimentary zooids are flask-shaped, with a conical projecting 

 h^'postome, 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 abo\e. 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 Pohqyora, Erri- 

 na, and Acanthopora these tentacular zooids are dispersed irregularly 

 amongst the alimentary zooids ; in Cri/ptohella, !Slt/la$ter erufjcsceus, 

 and AUopora they are arranged in a circlet around a centrally 

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

 The bases of these zooids communicate by large vascular offsets 

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

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

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



• Forh. Selsk. Christ. 1872, p. 115. 



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