The Pennatulidæ are regarded by naturalists as the most complicated or highly organised group of the Alcyonaria. Three different forms of polype build up the colony or sea-pen as it is called. There is a single very much modified and enormously large polype, without tentacles, forming the axis, a large number of ordinary Alcyonarian polypes (autozooids) arranged in the form of leaves, or simply scattered irregularly on the surface of the central polype, and a number of very small undeveloped polypes (Siphonozoids) without tentacles, whose function seems to be to pump water into the canals of the colony, and thus to keep up the circulation of water. [Illustration: FIG. 10.—Umbellula Güntheri. Nat. size. After Agassiz.] The deep-sea genus _Umbellula_ possesses a very long and delicate axial polype, and the Autozooids and Siphonozooids form a little cluster only at its extreme summit. The small number of these polypes and the very limited area over which they extend are the two most characteristic features of the genus. It would take me too far into the anatomy of the group if I were to add any further details; but I cannot pass on without noting that the whole structure of Umbellula shows that it is far more primitive and simple than the shallow-water genera. And, generally speaking, this holds good for all the deep-sea Pennatulids. In fact, we have here one of the rare examples of a series of genera, that can be regarded as a slightly modified ancestry of the shallow-water genera, that has been brought to light by the exploration of the abysmal depths of the ocean. We have seen, then, that of the Cœlentera, the only order that has a large proportion of its genera living in deep water, is the only one whose members all possess a stalk by which they fix themselves into the mud or sand at the bottom of the sea. It is not uninteresting to note, then, in passing on to the Echinoderma, that the stalked Crinoids, the only Echinoderms that can permanently fix themselves to the bottom, are nearly all found in deep water. Several years before the ‘Lightning’ was despatched on her memorable pioneering voyage, Vaughan Thomson had proved that the common feather star of the shallow waters of the British coasts passes through a stage in its development which resembles the fossil genera of the order in being provided with a stalk for attachment. But it was left for the naturalists of the ‘Porcupine,’ the ‘Challenger,’ the ‘Talisman,’ and other vessels employed in deep-sea researches to prove that adult stalked Crinoids are still living in nearly all parts of the world at the great depths of the sea.