Manchester Memoirs, Vol. Ix. (1916), 13 



about their powers of movement, but judging from their 

 anatomical structure it is clear that some kinds are a great 

 deal more mobile than others. Now in this Order we find 

 a series of forms beginning with the Veretillidae, which 

 are radially symmetrical, and ending with the Pennatulidae 

 which are bilaterally symmetrical. Between these two 

 extremes there is a number of families showing a gradual 

 increase in the intensity of the bilateral symmetry. 



The systematist who begins his studies with the 

 radially symmetrical Veretillidae finds the group exceed- 

 ingly difficult on account of the extraordinary variability 

 of the recognised species and genera, and the way they 

 run into one another. This great variability is seen in 

 the shape of the colony, the size of the zooids, the size 

 and shape of the spicules and in the colour. As an 

 example of this I may refer to the illustration given in 

 my memoir 5 of the spicules of three specimens of Caver- 

 nnlaria orientalis taken from the same locality in shallow 

 water at Amboyna. Attention may also be called to the 

 remarkable variations of Actinoptilum molle from the Cape 

 of Good Hope, described by Dr. J. Stuart Thomson in a 

 recent memoir published by this Society. 



In this family, too, we find that the axis which is 

 invariably present and of a definite kind in the bilaterally 

 symmetrical groups may be absent, imperfect or complete 

 in closely related species. 



Passing on to the family Kophobelemnonidae, which 

 shows the beginnings of the bilateral symmetry, we find 

 in some forms, and particularly in those species with the 

 bilateral symmetry less well developed, a similar varia- 

 bility. For example, in specimens of Sderobelemnon 

 Burgeri from Molo Strait the spicules have an extra- 



5 " Pennatulacea of the ' Siboga ' Expedition,'' fig. 9, p. 53. (In the 

 Press.) 



