124 



LOWER INVERTEBRATES. 



latter position in many allied genera, polyps are borne as in all corals. It commonly 

 happens that many of these polyps are aborted in character, those upon the stem 



especially assuming this form, so that there 

 is present a polymorphism of most simple 

 character. 



In Renilla, an extraordinary genus allied 

 to Pennatula, we have a still greater differ- 

 ence between the two kinds of zooids found 

 on its body, and a still better illustration of 

 the principle of polymorphism so well seen 

 in many of the jelly-fishes. The form of the 

 genus MeniUa departs widely from that of 

 almost all other Actinozoa. The body has 

 a thin, flat, kidney shape, from which hangs 

 a short, highly flexible hollow stem. There 

 is no hard axis such as is found in some 

 other Halcyonoids, and the body walls are 

 flexible throughout. The polyps are borne 

 upon one surface of the disk-like body, and one of 

 these, known as a Sauptzodid, has a prominent size 

 and specialized character. JRenilla, like Peniiatula, is 

 a free coral, and its attachment to the sand is of the 

 loosest nature as compared with the stony base of 

 CoralUmn and Isis. 



The family of TJmbei/- 

 LULiD^ is also of widely 

 aberrant and most inter- 

 esting character. The 

 several genera which 

 compose it are most 

 closely allied to the Pen- 

 natulidte, and are as a gen- 

 eral thing found in deep 

 seas. The genus Umbel- 

 hdaria\va,s described long 

 ago, and the accurate figure given of it remained, for many 

 years, the best and only account of the animal. Although 

 discovered in comparatively shallow water, the great explor- 

 ing exiDcditions of the past twenty years have again brought 

 the animal to light, this time from profound depths. 



In Umhellularia we have a long axis, more or less firmly 

 attached by one extremity, while from the other there 

 arises a cluster of polyps in the form of a terminal tuft. 

 These polyps are of two kinds. Some approach closely 

 the regular form of the Halcyonoids, while others resemble 

 the abortive zooids of Pennatula and Sarcodictyon. We 

 have here another expression of a law of polymorphism already pointed out in Henilla, 

 and already developed at length in our account of the Siphonophora or tubular medusae. 



Fig. 120. — Umbellularla granlandica, 

 natural size. 



Fig. 121. — Pteroides, sea-pen, one- 

 fourtli natural size; a, a single 

 individual, a little enlarged. 



