33 



remarks on the trno fislies, and I shall therefore pass on to the 

 consideration of the highest niollusca, the Cuttle Fishes and their 

 allies, the Octopus and the Eledone. 



I never think of this class of animals, known as Cephalopoda, 

 or head-footed, without reflecting on the causes which could have 

 led to the extinction of three of the families entirely, and the 

 proximate extinction of a fourth family of the class. These four 

 families, of which 2,333 species are known in the fossil state, are 

 reduced in these days to the six species of Nautilus only. 



Their places seem, in the present day, to be taken by the lower 

 Mollusca, such as the water-breathing shelled Prosobranchiate 

 Gasteropoda, of which about 8,500 recent species are known, and 

 only 5,800 fossil ; whilst of the Gasteropoda, which breathe by 

 lungs, only 588 are known as fossil, against 6,335 recent. 



The conditions of the struggle for existence must have very 

 much changed at different epochs of the world's duration and 

 favoured races, responding to their environment, became domi- 

 nant, to give way when the environment was changed, to others 

 better fitted under the new conditions to survive ; so that, by the 

 plasticity of natural objects, the world, in all the geological 

 changes it has passed through, appears always to have had an 

 abundance of ever-varjdng life. 



Two genera of these Cuttles have survived in the aquarium. 

 Octopus and Eledone, and they have been the most attractive 

 creatures exhibited. It is much to be regretted that when in 

 health they hide in the rocks, and are with difficulty made to 

 exhibit themselves. 



These two species crawl over rocks by means of their eight 

 arms, so called, but in no way homologues of the arms of verte- 

 brates, and move rapidly through the water by means of the 

 forcible expulsion of water through a siphon, and also by a con- 

 traction of their tentacles, like the quick shutting of an umbrella. 



The mode in which the suckers are attached and detached when 

 the animal crawls in the glass is well worthy of observation. The 

 creature makes a partial vacuum by drawing from the glass the 

 central part of each sucker, and when it wishes to detach the 

 sucker, it simply pushes the central part forward and draws back 

 the edges of the disk, detachment at once takes place. 



Sepia, the species which produces the pounce used for taking 

 out ink blots, has lived in the aquarium a few days, and I had 

 the delight of seeing it. The changes of colour when the creature 



