1891.] Mechanical Origin of Structure in Pelecypods. 21 
circular in section at any one horizon, and subconical, cup-shaped, 
or globose in its entirety, as are the forms which we are here con- 
sidering. To strengthen the walls of around organism we might 
have solid accumulation, vesicular accumulation, or perpendicular 
partitions arranged at right angles to the supported wall. Such 
mechanical supports are characteristic of attached Rudiste and 
other Pelecypods, Ccelenterates, Cirripeds, and some worms. 
As all sides of the periphery of an attached organism are equally 
exposed to food supply, danger, etc., the organs, as tentacles, 
nerve centres, and eyes would gradually tend to become situated 
at all points on the periphery, or radially. It is well known that 
the external parts of an animal are more easily modified than the 
deeper-seated parts. It is also known that the modification of 
deeper-seated parts may be progressively produced from without 
inward, or centripetally. If we have an attached animal that is 
round, with some organs arranged on the periphery, a common 
case, then further modification or development of such organs or 
parts would tend to be produced centripetally, or radially, for the 
centripetal variation of a round form would, as a mechanical 
necessity, be radial variation. In the Hydrozoa there is a strong 
proof of the correctness of this view. Passing from Protohydra 
to Hydra, to the hydroid stage of Aurelia, then to the free 
medusoid stage of Aurelia and other Discophores, we find pro- 
gressively a more and more perfect radial symmetry built up 
centripetally. A similar comparison may be made in the develop- 
ment of corals. 
Many permanently attached forms of bilateral classes of animals 
assume a considerable degree of radial symmetry; and, conversely, 
many free locomotive forms of radiated classes assume a consider- 
able degree of bilateral symmetry. As bilaterality is a feature 
induced and progressively built up by the conditions of free loco- 
motion, it is believed that radiality is induced and progressively 
built up by the conditions of permanent fixation, with its attend- 
ant influences of environment.” ; 
13 Since this paper was in type my attention has been called to the fact that Haeckel, in 
as the 
his Gastrzeatheorie, considers the sedentary life of ancestral forms of the os 
mechanical efficient cause of their radial symmetry. 
