136 



LECTURE Vn. 



fabrication of the coral islands and reefs belong to the present group, 

 and have essentially the organisation of the sea-anemony, which has 

 just been described; but those of the genera Madrepora proper, 

 AstrcBa, and Oculina have not more than twelve tentacles. 



To the eight-armed division of the anthozoic Polypes belong those 

 species which have an internal ramified calcareous, or flexi- 

 ble, or jointed axis, e. g. as the red coral, (Jig. 64, c), the 

 gorgonia, and the isis. To this division likewise belongs 

 our common Lohularia digitata, or " dead-man's-toes," in 

 which the hard axis is wanting ; and the phosphorescent 

 Sea-pens, the Veretillum, and other Pennatulidce, in which 

 it is in detached pieces. In all this division the polypes, a, 

 are retracted into the soft enveloping tissue, b, not into 

 calcareous cells. 



These examples of the compound Anthozoa differ from 

 the compound Hydrozoa in having an internal instead of an external 



skeleton. The body of each polype 

 {Jig. Q5.) is relatively longer than 

 in the ActinuB ; the prehensile ten- 

 tacles (a, a) are broad and pinnate, 

 or with scolloped margins.. At the 

 centre of their base is situated the 

 mouth (i), which leads to a straight 

 membranous alimentary cavity, fixed 

 by vertical septa {d, d), called meso- 

 gastric folds, to the external inte- 

 gument: which septa are continued 

 down the general visceral cavity. 

 The digestive canal communicates 

 with this cavity by a small orifice (c), 

 at its inferior part. Sea-water passes, 

 probably with chyle, through this 

 aperture, to circulate over the ex- 

 tended ciliated surface of the abdo- 

 minal cavity and the interior of the 

 tentacles. The hepatic cells of the gastric parietes present a yel- 

 lowish colour in Alcgotiium, and a brown colour in Veretillum. 



The contractile power of both the Polypes and of their common 

 fleshy base in the softer compound kinds is considerable. In the 

 Alcyonidium the lower half of the polypary is of a firm coriaceous 

 texture, the upper half is softer and divided into branches, the sum- 

 mits of which arc crowned with polypes of the structure shown in 

 fig, 65. These polypes, when touched, not only shrink into their 



Polype of the Alcyonydium elegans, 

 dissected and magnified. 



