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 molluses, 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, ete. 
Around the vertebrate axis a modified radial system seems to 
prevail. Owen and Humphry advocated this, although not in so 
