1899] CONSIDERATIONS CONCERNING SYMMETRY 103 



to circumference may in part be responsible for the radiate character 

 of the tubes, but the other forces already alluded to in other structures 

 cannot be lost sight of in this connection, nor the fact that the contrac- 

 tion and dilatation of the umbrella favours the circulation of fluids in 

 certain directions. 



Passing over the tunicates, which may be radial in colonies and 

 bilateral in individuals, the worms, arthropods, and vertebrates may be 

 noted. A bilateral symmetry is here evident enough. Not only in the 

 early forms, but in the adult life of many of these and molluscs, a 

 disguised radiate symmetry seems to prevail. 



The chief axis of the yolk sac in the chick may be regarded as an 

 axis of symmetry in the young animal. There may or may not be the 

 remains of an apparently azygous organ, but a radiating system of 

 alimentary tubes is easy to see in some animals, and a like arrangement 

 in the nervous and vascular systems in others that are easy to group 

 with a central axis. The paired ganglia above and below the anterior 

 part of the alimentary canal in worms and arthropods, and the three 

 pairs of ganglia in the molluscs, may also be regarded as an exaggerated 

 radiate symmetry. Then the alimentary canal has been looked upon as 

 forming the central axis of the system, an axis often strengthened by 

 lime or chitin, deposited or formed in a tissue derived from without ; 

 the cells also that form bone are probably derived from the outer 

 embryonic cell layer. The vascular system consists chiefly of four 

 tubes in some worms (dorsal, ventral, and lateral). The nervous system 

 may occupy the sides in the central part of the body, or dorsal and 

 ventral cords may be both present in the same animal. This bilateral 

 symmetry might be regarded as a modified kind of quadrilateral sym- 

 metry. The special development of certain parts emphasizes the former 

 variety. The dorsal tube feet in some holothurians are dummies, whilst 

 in others are three rows of tube feet on the ventral surface, and two on 

 the dorsal. There are indications of a bilateral symmetry in the 

 interior. The enamel of the teeth is derived from a portion of the 

 invaginated skin in the vertebrates ; so, if, passing over the early stage, 

 it be desirable to take the alimentary canal as the axis of symmetry, 

 some ingenious attempts may be made to give force to the assumption. 

 The position of the primitive mouth will not then escape attention, 

 nor will the fact that the sympathetic has a good district in the 

 alimentary canal. If this study be pushed as far as one can decently 

 go, and the ground changed to the spinal canal and cord, then a most 

 instructive method of comparison may be noted, viz. on the dorsum a 

 canal, a nervous cord around it, and the appropriate serous membrane, 

 blood vessels, muscle, and bone ; and, on the ventral part, the intestinal 

 canal, a sympathetic neuro- muscular system, serous membrane, 

 vessels, etc. 



Around the vertebrate axis a modified radial system seems to 

 prevail. Owen and Humphry advocated this, although not in so 



