Evolution and Symmetry in the Order of the Sea-pens. 109 



symmetrical, on the other hand, are usually either sedentary iu habit, 

 or have a floating or drifting habit with the aquatic plankton. 



There is clearly an intimate association between the symmetry and the 

 habit. An animal that is active and moves rapidly from place to place in 

 search of food or shelter must be bilaterally symmetrical. The animal has 

 been constructed by the forces of Nature in the same way as man constructs 

 his railway trains, steamships, and aeroplanes, with a view to rapidity of 

 progress combined with economy of power. 



An animal that is sedentary or drifting in habit, one that does not use its 

 muscles for driving its body through the water, through the air, or on the 

 surface of the earth, does not need this exquisite balance of parts on a 

 median vertical plane. It may be any shape that is not inconsistent with 

 the conditions of the environment, as we see in variations of shape and rami- 

 fication of corals and sponges. But, provided that the forces that act upon it 

 and the food upon which it preys come to it with equal intensity from all 

 directions, it has a tendency to assume a more or less perfect radial 

 symmetry. 



"vVe can see the influence that habit has had in determining the symmetry 

 of the body in the evidence of phylogeny afforded by the embryonic develop- 

 ment of several kinds of animals. 



Thus, the Crustacea are, as a rule, animals with perfectly bilateral sym- 

 metry of the body, and they are bilaterally symmetrical from the early 

 embryonic stages until adult life is reached, and all the time they are active 

 and free. But in the order Cirripedia we find a series of forms in which the 

 bilateral symmetry of the external shell is gradually obscured until we reach 

 the almost perfect radial symmetry of such a genus as Coronula. The 

 nauplius and the cypris larval stages of these Cirripedes are free and, like 

 other Crustacea, bilaterally symmetrical, and it is only in the adult and 

 sedentary stage that the tendency to the assumption of a secondary radial 

 symmetry shows itself. 



Again, in the Echinodermata we find a dominant radial symmetry, asso- 

 ciated in many of the Crinoidea with a sedentary habit, and in the forms 

 that are not strictly sedentary the movements are slow and indeterminate in 

 direction. The embryological evidence that the Echinoderms have passed 

 through, in their phylogeny, firstly a free bilaterally symmetrical stage is 

 almost conclusive ; but it is even of greater interest to find that in some of 

 the Holothuroidea, a group in which the movements in a definite direction of 

 the body are assisted by a more powerful development of muscles in the body 

 wall, several genera — such as the Elasipoda — exhibit a very pronounced 

 return to a bilateral symmetry of the body. 



K 2 



