212 ACTINOZOA. 



polyp 



In other 



in Virgularia, the sclerobasis of which is rioid 

 tapering towards its extremities, and densely cal- 

 careous {fig. 3 7, a). In the true Sea-pens, forming 

 the genus Pennatula and its near allies, the pin- 

 nules are very conspicuous, and so modified as to 

 arrangement and comparative size that the whole 

 mass presents a striking resemblance to a bird's 

 feather. The proximal end of the coenosarc, often 

 for nearly half its length, is bare of pinnules or 

 polypes, appearing swollen and fleshy. 

 Pennatulidw the entire coenosarc is club-shaped, 

 without any pinnules, the polypes being irregularly 

 scattered, as in Veretillum, or arranged in longi- 

 tudinal rows on part only of the surface, as in 

 Kophobelemnon. In Renilla a comparatively 

 short coenosarc expands distally to support a 

 smooth, symmetrical, kidney-shaped disc, from 

 the free surface and edge of which the scattered 

 polypes arise. This aberrant genus appears to 

 want a sclerobasis, the interior of the stalk and 

 disc being hollow, and in free communication with 

 the cavities of the polypes, so that the animal 

 possesses the power of largely increasing its 

 dimensions by allowing itself to become expanded 

 by the ingress of the surrounding sea-water. 



Like the members of the preceding families, 

 many Pennatulidce are liable to have their soft 

 structures strengthened by the deposition of spicu- 

 lar concretions. 



The Sea-pens still further interest us by reason 

 of their beautiful phosphorescence. What Agassiz 

 has observed in Renilla is probably true of the 

 entire group. « It shines at night with a golden 

 green light of a most wonderful softness. When 



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