The development of the Cephalopoda affords at present no better evidence that the metamerism is secondary.
As in Cephalopoda (and possibly other Mollusca) water can be introduced through the nephridia into this space.
It has been discovered in seaweed; in the blood of certain Cephalopoda and Ascidia as haemocyanin, a substance resembling the ferruginous haemoglobin, and of a species of Limulus; in straw, hay, eggs, cheese, meat, and other food-stuffs; in the liver and kidneys, and, in traces, in the blood of man and other animals (as an entirely adventitious constituent, however); it has also been shown by A.
The chief types of Mollusca were already differentiated at the beginning of the geological record, and the metamerism which occurs in the Cephalopoda has been evolved within the limits of that class.
In Polyplacophora there are eight on each side (8.I.8); in Scaphopoda two on each side (2.I.2); in almost all Cephalopoda three on each side (3.I.3); in Gastropoda the number varies very much in different subdivisions.
The Cephalopoda can be derived without much difficulty from the schematic Mollusc, if we assume that some metameric repetition of organs has occurred, as explained above in reference to the coelom.