88 MR. R. aARNER ON THE SHELL-BEARINa MOLLUSCA. 



branchiata, be observes that any number of individuals of Cyclas 

 may be examined, and young fry will be found in the branchial 

 laminae in all ; that all Oysters have ova, and also all individuals of 

 Tecten maximus, the subpedal mass being visibly composed of an 

 ovary and a testis. He is obliged to believe that one species of 

 British Anodon is universally oviferous. But the common Edible 

 Cockle appears to have the individuals of different sexes, and the 

 same may be said with regard to Mytilus edulis and Patella. 



The spermatozoa in the Cockle are oblong and a little curved, 

 and torulated, as it were, whilst they are pear-shaped in My- 

 tiltis ; they are also extremely minute, and their appendages must 

 be very fine, for with a power magnifying 500 diameters they are 

 scarcely to be seen. 



In the shell of a Patella, Emarginula, or Haliotis, we have 

 the two conjoined valves of a lamellibranchiate mollusk; and 

 through such forms as Calyptrcea, Hipponyx, Navicella, and Ne- 

 rita, we arrive at the ordinary form of the gasteropod with its 

 operculum. 



Then follows a disquisition on the progressive tendency to a spiral 

 geometry in these animals, due to a varying plan of conformation, 

 and not to the force of the heart, there being generally an atrophy 

 of the left side of the body. In Nautilus and Aryonauta, the shell 

 and mantle are reversed in position to what they are in the Gaste- 

 ropods, whilst Sepia and Hyalcea agree rather with the latter. 

 The symmetrical shell of the lower Gasteropods undergoes a 

 lateral torsion in the higher, spiral forms, to become again 

 symmetrical in the Cephalopoda. The branchiae in Patella retain 

 a position analogous to that of the same organs in the Lamelli- 

 branchiata; in some Chitons they have a tendency to retract 

 towards the anus, as in Doris ; in Fissurella they waste at the 

 sides and become developed above the neck, as in the spiral 

 Gasteropods ; but in them, the right branchia, and right side of 

 the mantle are principally developed. From this torsion arises 

 the form and spire of the shell. In Aplysia, where the bran- 

 chial fissure is far back and to the right side, the right respiratory 

 nerve preserves a superior position, and passes backwards to form 

 its ganglion at the front of the branchial opening ; the left, on the 

 contrary, passes under the oesophagus to form a second ganglion, 

 not mentioned by Cuvier, behind the first. In the more spiral 

 Gasteropod the torsion is greater ; the right nerve, for instance, 

 mounts upwards over the digestive canal to form its ganglion 

 quite in the left flank, whilst the left goes below the digestive 



