CHAP. X] CONDUCTING TISSUES. 205 
The greater celerity with which the impulse is transmitted 
down the long exterior tentacles than across the dise may be 
largely attributed to its being closely confined within the 
narrow pedicel, instead of radiating forth on all sides as on 
the dise. But besides this confinement, the exterior cells of 
the tentacles are fully twice as long as those of the disc; so 
that only half the number of transverse partitions have to 
be traversed in a given length of a tentacle, compared with 
an equal space on the disc; and there would be in the same 
proportion less retardation of the impulse. Moreover, in 
sections of the exterior tentacles given by Dr. Warming,* 
the parenchymatous cells are shown to be still more elon- 
gated; and these would form the most direct line of com- 
munication from the gland to the bending place of the 
tentacle. Ifthe impulse travels down the exterior cells, it 
would have to cross from between twenty to thirty trans- 
verse partitions: but rather fewer if down the inner paren- 
chymatous tissue. In either case it is remarkable that the 
impulse is able to pass through so many partitions down 
nearly the whole length of the pedicel, and to act on the 
bending place, in ten seconds. Why the impulse, after 
having passed so quickly down one of the extreme marginal 
tentacles (about >} of an inch in length), should never, as 
far as I have seen, affect the adjoining tentacles, I do not 
understand. It may be in part accounted for by much 
energy being expended in the rapidity of the transmission. 
Most of the cells of the disc, both the superficial ones and’ 
the larger cells which form the five or six underlying layers, 
are about four times as long as broad. They are arranged 
almost longitudinally, radiating from the footstalk. The 
motor impulse, therefore, when transmitted across the disc, has 
to cross nearly four times as many cell-walls as when trans- 
mitted in a longitudinal direction, and would consequently be 
much delayed in the former case. The cells of the disc 
converge towards the bases of the tentacles, and are thus 
fitted to convey the motor impulse to them from all sides. 
On the whole, the arrangement and shape of the cells, both 
those of the disc and tentacles, throw much light on the rate 
and manner of diffusion of the motor impulse. But why the 
impulse proceeding from the glands of the exterior rows of 
* ¢Videnskabelige Meddelelser de la Soc, d’Hist. nat. de Copenhazue,” 
Nos, 10-12, 1872, woodcuts iv. and v. 
