GASTROPODA ANISOPLEURA. 479 



The respiratory organs are represented either by ctenidia, or by the 

 surface of the branchial cavity formed by the mantle, or else by secondarily 

 formed processes of the body wall. Processes of every kind are absent, 

 and the general surface of the body is respiratory in certain Non-Palliata 

 the Haplomorpha, e.g. Phyllirhoe. Two ctenidia are present only in the 

 Haliotidae, and Fissurellidae among Zygobranchia. In all other A nisopleura 

 there is but one. But in some Azygobranchia the osphradium is large, 

 thrown into folds, and is generally taken for a second but reduced cteni- 

 dium (parabranchia). The single persistent ctenidium is on the left side of 

 the anus, and therefore represents the original right ctenidium. The 

 Limpets (Patellidae) among Zygobranchia have the two ctenidia quite rudi- 

 mentary, forming the capito-pedal organs of Lankester. The single cteni- 

 dium of palliate Opisthobranchia is also lost in the Phyllidiobranchia. In 

 both cases vascular lamellae or pallial gills are developed in the mantle fur- 

 row, forming a complete series in the former of the two groups (Patellidae}, 

 a right and left row in the latter. A circle of pinnate folds surrounds the 

 anus in certain Opisthobranchia Non-Palliata, i.e. the Pygobranchia, e.g. 

 Doris. It is probable that they are secondary processes, not true ctenidia, 

 and in certain members of the same sub-order, as well as in the sub-order 

 Ceratonota of Non-Palliata, cylindrical hollow processes of the body walls 

 the cerata are developed, more or fewer in number, and extending in 

 lateral rows on each side of the body. Into these cerata liver caeca may 

 extend (supra}. In one group of Azygobranchia the Pneumochlamyda, 

 e.g. Cyclostoma and in the Pulmonata, the roof of the branchial cavity 

 formed by the mantle has taken on a respiratory function, and is traversed 

 by a network of vascular sinuses. In the genus Ampullaria (Azygobranchia 

 Holochlamydd} from the fresh-waters of tropical America, Africa, and East 

 Indies, one side of the roof of the branchial cavity is respiratory, but a 

 ctenidium is present as well. 



Two nephridia are found in some Zygobranchia (Fissurella, Patella}, 

 but not in others (Haliotis}. One of the two the left, i.e. the primitive 

 right nephridium is rudimentary. It appears to be the one that is aborted 

 in other Anisopleura, and not the right (primitive left), as is generally 

 maintained. In Haliotis, and still more in Turbo, where the organ is on 

 the right side of the body, its duct crosses the intestine and opens on its 

 left side. In other instances the organ itself appears to undergo a greater 

 or less change of position 1 . The nephridium consists of a urinary chamber 

 opening externally, of secreting and usually branched acini, the calibre of 

 which is variable, and a canal which connects the pericardium and the 

 urinary chamber. 'This chamber is primitively small, e.g. Fissurella, and 



1 The point is one which requires further research. It is possible that the Anisopleura do not 

 all agree. See Haller, M. J. xi. 1886, p. 26. In Paludina the nephridial duct is to the right of the 

 rectum, between it and {he vagina. 



