208 INVERTEBRATE ANIMALS. 
and the water is forcibly expelled through the funnel, which is 
often furnished with a valve, allowing the passage of water out- 
wards, but preventing its entrance inwards. By a repetition of 
this process both respiration and locomotion are simultaneously 
effected, forthe jets of water expelled from the funnel by their 
reaction drive the animal in the opposite direction. In this case, 
therefore, as in many others, the more active the animal is, the 
more perfectly is the respiratory process carried on. 
Fig. 111.—1. Octopus carena (male), showing cyst in place of the third arm. 2. An 
individual, more developed, with the metamorphosed arm liberated from the cyst 
(a). (After Woodward.) ‘ 
The nervous system is formed upon essentially the same plan 
as in the other J/o//usca, but the cerebral ganglia are protected 
by a cartilage, which is to be regarded as a rudimentary skull. 
This structure, therefore, is a decided approach to the Vertebrate 
type of organisation. 
