28 MOLLUSCOUS ANIMALS. 



Order 9. Cyclobrancliiata is distinguished by the branchias, like little 

 leaflets or pyramids, attached in a cordon under the border of the 

 mantle, the sexes being united in the same individual. The animals 

 are the lowest of the great Pectinibranchiate group, to which, however, 

 they certainly belong. The technichal character resembles that of 

 Inferobranchiata, but the organization is very different. The last four 

 orders having the branchiae in the anterior portion of the body are now 

 generally combined under the name of Prosobranchiata. Perhaps, it 

 would be still better to make Cuvier's name Pectinibranchiata, which 

 well expresses the common character, embrace them all. They con- 

 stitute the specially typical group among the Gasteropoda, and their 

 further sub- division is a subject of great interest. 



Some late writers on the subject combining, as already explained, all 

 the orders related to Pectinibranchiata under the name of Prosobranchi- 

 ata, unite also Tectibranchiata with Nudibranchiata and Inferobranchiata 

 under the common name of Opisthobranchiata, thus reducing the whole 

 class to four orders. This, however, is liable to great objection from 

 the decidedly distinct characters of Nudibranchiata, which seem mani- 

 festly to claim for it rank as an order. If we only reduce Inferobranch- 

 iata to Tectibranchiata, and give its natural and just extension to 

 Pectinibranchiata, we have five well marked orders of which it seems 

 evident that Nudibranchiata occupy the lowest place, and Pectini- 

 branchiata, that which represents Gasteropoda among the orders. But 

 Nucleobranchiata are remarkable for the high development of their 

 organization, and for resemblance to the higher classes, Pteropoda and 

 Cephalopoda, we may therefore place them first. Pulmobranchiata will 

 without question stand second, and then we have the five orders reduced 

 to their natural series. In forming his orders Cuvier relied on the 

 different position of the branchiae, and on the comparative development 

 of the reproductive system, the former being admitted as the leading 

 character. Allowing the correctness of his opinion on each point, there 

 are other considerations which justify the combinations now proposed, 

 since, for example, the branchiae are essentially of the same kind in 

 the whole of the Pectinibranchiata in the extended sense we have given 

 to the term, and there are other points of resemblance uniting them as 

 one great natural group, whilst the separation. or union of the sexual 

 distinctions may assist in determining the comparative rank of the 

 families, though not allowed to multiply orders by breaking up a great 

 natural assemblajxe. 



