158 



COMPARATIVE ANATOMY 



CHAP. 



useful, the return to symmetry in the pallial coni[)lex and mantle cavity or fold 

 would be advantageous, since then symmetrical growth of the shell could take 

 place. If the dift'erence between the maximum and minimum growth in height 

 is but slight the shell would be but slightly coiled, and if the peripheral growth 

 is pronounced, while the growth in height is insignificant, a flat cup-shaped shell 

 would result {Haliotis, Emarginula, Fissurdla, Patella, etc.). 



13. 



Chiastoneury only takes place when the original right half of the pallial complex 

 crosses over to the left of the median line anteriorly. 



This crossing of the line of symmetry has actually taken place in the Proso- 

 hranchia. The original right gill in them lies quite to the left of the mantle cavity. 

 In the Azygohranchia and Monotocardia the hind-gut with the anus has at the same 

 time become displaced into the right (original left) narrower gill-less half of the 

 mantle cavity, which, however, is still spacious enough to contain the rectum. The 

 Prosobranchia are streptoneurous. 



In the Tcctihranchia and Opisthohrancliia the pallial comjilex is found on the right 

 side of the body, and has nowhere crossed the median line anteriorly. There is 

 therefore no chiastoneury among the Opisthobranchia, i.e. their visceral connectives 

 are never crossed.^ 



In the Pulmonata the pallial complex has shifted far forward, but it has not 

 passed the middle line with anj^ organ which, drawing the parietal ganglion and the 

 visceral connective with it, could have brought about chiastoneury. For the left 

 (original right) gill, the only one elsewhere retained, ^disappeared (apparently very early) 

 in the Pulmonata. The osphradium, which is retained in acpiatic Pulmonata, is the 

 original right, and still lies on the right side. In considering the arrangement of 

 the nervous system, it is really immaterial whether we assume that the hind-gut 



has shifted back to the right 

 secondarily, and the osphra- 

 dium moved to near the re- 

 spiratory aperture, or that 

 the hind-gut never reached 

 the median line, and that 

 the osphradium never passed 

 over it. The Pulmonata are 

 thus euthyneurous. 



14. 



We saw, in paragraph 

 3, that with a strongly de- 

 veloped visceral dome and 

 posteriorly placed pallial 

 complex, a shell inclined 

 forward or coiled forward is 

 an impossibility for a rep- 

 tant Gastropod. But such 

 a shell is not an impossibility for an animal which does not creep. For example, 

 in a swimming animal, whose shell, partly filled with air, serves as a hydrostatic 

 apparatus, there is no reason why a much developed visceral dome and shell should 



^ Except in Actoion, an exception which makes it probable that in the Opistho- 

 branchia the pallial complex has secondarily returned from an anterior position. 



Fio. 137.— Nautilus, diagram, do, Dorsal ; re, ventral ; vo, 

 anterior; hi, posterior. 



