Respiration in Invertebrate Animals. 87 



or smooth, type of the lamina in the Paludinidse and the pHcated 

 form of the organ which prevails probably throughout the Litto- 

 rinidan family. An opportunity of examining minutely a recent 

 specimen of Valvata has not yet occurred to the author. 



In the genus Littorina, the last turn of the shell is very large 

 relatively to the second and the third. By this criterion, the 

 capacity of the respiratory chamber may be estimated. It pre- 

 sents a considerable size in these mollusks compared with the 

 bulk of the entire body. The augmented dimensions of the 

 cavity are due to the increased volume of the contained organs 

 (fig. 1). The branchia (fig. 1 a, a) is highly developed, and 

 occupies a large share of the cavity. It extends from the hind- 

 most boundary to the root of the siphon. It lies obhquely from 

 left to right along the roof of the chamber. Viewed as a whole, 

 it will be observed to consist of two halves (fig. 2 a, a), which 

 are divided by a clearly marked line extending from one extreme 

 of the organ to the other. One of these halves (fig. 3 a, a) corre- 

 sponds with the dorsal or cartilaginous borders of the leaves, 

 and consists of a series of parallel unbending rigid liues or fila- 

 ments ; the other half (fig. 3 ^), more wavy, flocculent, soft and 

 flexible, coincides with the membranous portion of the laminae. 

 To the left of the gill is situated a peculiar gland (fig. I d) 

 having a bipectinate appearance, less developed in this mollusk 

 than in Buccinum, and which Dr. Sharpey, and after him all 

 systematic malacologists, has described as a double, though rudi- 

 mentary gill. It will be shown that it is a true gland. To the 

 left of the branchia there lies a large glandular mass, which is 

 always enveloped in viscid mucus, and which exhibits a leafy 

 or laminose structure. On the reflected roof (as shown in 

 fig. 1), still further to the left, is observed another glandular 

 mass {e), which some anatomists have described as the renal 

 organ ; and, lastly, a duct which belongs to the reproductive 

 system {j). The structure of these glands will be discussed 

 on another occasion : the branchia only will be at present 

 described. Powdered Lycopodium strewn lightly over the fresh 

 organ will move in one definite direction; namely from the 

 right, or cartilaginous border, to the left, or membranous 

 (arrows, fig. 1 a, a). This current, examined more closely, 

 will be found to be subdivided into as many rivulets as there 

 are spaces between the leaves of the entire organ. Of course 

 these superficially indicated currents are but the edges of ver- 

 tical sheets of water which are in the act of traversing the 

 spaces between the laminae. These cuiTcnts are impelled by two 

 forces, one of which is due to the action of the numerous minute 

 muscles fixed to the cartilaginous margins of the laminae (fig. 14 

 c, d)j whose office it is to furl and unfurl, approximate and 

 separate the individual leaves. By this contrivance a mechanical 



