14 HlCKSON, Animal Symmetry. 



ordinary variability in shape and in size (V. " Siboga " 

 Monograph, fig. 18, p. 84). But when the systematist 

 comes to deal with some of the families that show a 

 pronounced bilateral symmetry he finds that the species 

 are far less variable as regards certain specific characters, 

 and the species are well defined. In the genus Pennatula, 

 for example, such species as Pennatula Murrayi, P. 

 Naresi, P. grandls, and others, are perfectly well defined, 

 and can be easily identified. The spicules taken from a 

 particular part of the colony have a constant form — the 

 3-flanged spindle — and within certain limits a constant 

 size. Moreover, the axis, the arrangement of the zooids, 

 and even the colour, are far less variable than they are in 

 the Veretillidae. Similarly with the genus Scytalium, the 

 spicules are extraordinarily constant both in size and 

 shape, and the few species are comparatively well defined. 



Of course there are difficulties even in these bilaterally 

 symmetrical Pennatulacea, witness the variability in the 

 shape of the leaves of Pennatula pliosphorea, and the 

 variability as regards certain characters of the genus 

 Pteroeides, but these difficulties are trifling compared 

 with those that occur in the radially symmetrical groups. 



In these few pages I have summarised the general 

 results of many years work on the structure of the Pen- 

 natulacea, but a great many more illustrations could be 

 given to indicate the way in which the gradual change 

 from a radial to a bilateral symmetry has been accom- 

 panied by a decrease in the variability of certain structures, 

 and the gradual differentiation of smaller discontinuous 

 types of colonies which are known as species. 



We are still in ignorance of the extent to which the 

 assumption of bilateral symmetry is accompanied by an 

 improvement in co-ordinated movements of the colony as 

 a whole, but speaking generally we do find that the 



