248 SHELLS AND SHELL-FISH. PART I. 



gills are on the back, with long pectinations on one 

 side : in the first they are covered by an oviform shell, 

 and in the latter by a convex plate, resembling half a 

 bivalve. In the three aberrant divisions, how^ever, the 

 branchia are lateral. Phyllidea has them on both sides ; 

 Pleurobranchus and Gasteropteron, on the right only ; 

 and, as Cuvier says that this latter genus " appears to 

 be an Akera with the foot developed into broad wings," 

 we consequently return again to the Bullince. If we 

 look, on the other hand, to the presence or absence of 

 the testaceous covering, the same circular course of the 

 groups can be demonstrated. The pre-eminent type, 

 seen in the genus Bulla, has the shell so perfect as 

 sometimes to be capable of receiving the greater part of 

 the animal, and it is always more or less convolute. In 

 the sea hares or Aphysiance, however, it is a mere ru- 

 diment, like one half of a bivalve shell; while in the 

 three aberrant types, even this vestige is totally wanting. 

 Some of Cuvier's genus Akera, he observes, have no 

 shell whatever, or only a vestige of one, yet with the 

 mantle of Bulla ; and as he insists, in another place, on 

 this genus being the closest to Gasteropteron, we come 

 to the same results as by studying the variation of the 

 branchia. The argumentum ad verecundiam may thus 

 be advantageously employed, though we generally prefer 

 resting our theory on the simple facts brought before 

 the reader. 



(232.) And yet, although there is evidently a cir- 

 cular disposition among the Tectibranchia, and a strong 

 affinity to the Cyclobranchia on one side, and to the 

 Fteropoda on the other, their affinity with the Gas- 

 teropoda is not so evident, — at least, if we consider the 

 BullcB as standing at the head of the tribe. This 

 point, however, cannot be well decided, until we know 

 the animals of such shells as Bulla naucum, Aplustra 

 pulchella *, &c. There can be no doubt, that between an 

 animal which can recede entirely within its sheU, and one 

 which envelopes it (as Akera, &c.), there must be many 



.. * Bulla aplustre Linn. 



