634 SUMilAKY OF CURRENT RESEARCHES RELATING TO 



discussion. It is however to-day pretty generally acknowledged that 

 the type of radial symmetry must have preceded that of bilateral 

 symmetry. Two important views are current as to the origin of a 

 bilateral foiTQ of symmetry, such as is presented by the group Vermes, 

 from a radial symmetry such as we find in the Ccelenterata. The 

 simplest view, of which Eay Lankester is an exponent, is as follows: — 



Starting with a radially symmetrical larva, this view supposes that 

 the two forms of symmetry arose with reference to the stationary or 

 locomotive life of the animal. On the one band, the stationary animal 

 retains its primitive radial symmetry, and grows into a radial adult. 

 On the other hand, the locomotive larva is modified by its free life. 

 Its growth, in order to give greater freedom of motion, results in an 

 elongation of the body in a direction parallel with its axis. Such a 

 long cylindrical body would of necessity soon develope swimming 

 organs ; and these swimming organs, in order to give greater steadi- 

 ness of motion, and prevent an inconvenient revolution of the body, 

 would appear in such a position as to give the animal an upper and 

 an under surface, and consequently a bilateral symmetry. With the 

 continued elongation of the body the digestive tract, which at first 

 ended blindly, would also elongate, and finally acquire a posterior 

 opening at a position directly opposite the mouth. This view, then, 

 supposes the body of the radiate animal to elongate in the direction 

 of its long axis, and a bilateral symmetry to arise in reference to the 

 organs of locomotion. 



A second view, advanced by Balfour, while based on the same 

 f andamental principle of stationary and free life, sujiposes the change 

 to take place in a difterent fashion. This view supposes that the 

 growth of the free living radiate form resulted in an elongation, not 

 in the direction of the axis of the animal, but rather at right angles 

 to this axis. This places the mouth of the animal, from the first, not 

 at one exti-emity, but on one side, which therefore becomes very early 

 the ventral side. The swimming organs afterwards arose in reference 

 to the already indicated bilateral symmetry. 



These two views are fundamentally ditferent. Besides affecting 

 our belief as to the manner in which bilateral symmetry arose, the 

 acceptance of one or the other is the foundation of our understanding 

 of the homologies which are to be found in the two groups. 



Evidence for the one or the other of the views is to bo looked for 

 in embryology ; but very few animals give an opportunity for such 

 research, owing to secondary changes which have acted upon the ova 

 and the embryos. For this reason no direct evidence has been 

 hitherto obtained. At Beaufort, during the last summer, some work 

 was done upon Tlialassevia, a species of worm which possesses a very 

 primitive development, and enables a direct study of the origin of 

 bilateral symmetry from radial symmetry to be made. The results of 

 the observations were satisfactory upon the point in question, and 

 showed that, as far as this group of animals is concerned, the second 

 of the above views, viz. that of Balfour, is in all essential resj)ects 

 correct. The radially symmetrical gastrula elongates nearly at right 

 angles to its long axis, and gives rise to a bilateral larva, of which 



