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118 ANATOMY AND LIFE HISTORY OP MOLLUSCA. 



they enclose a space (intra-branchial space) ; when the gill-fi laments 

 grow together, this space traverses the whole of the gill-plate, and 

 communicates with the exterior by means of the clefts between 

 the filaments. The water which enters by these clefts is collected 

 into a canal at the point where the plate is attached, and is carried 

 by it to the hinder end of the body. There are chitinous rods in 

 each of the gill-filaments, which form a special organ of support." 

 (Gegenbaur.) This quotation will be a sufficient explanation of 

 the observations which follow. 



In all the Mollusca that I have examined, I have seen scarcely 

 any exception to this general type of structure ; though there is 

 one species in which I have not been able to find it. This is 

 Cerithium ebeninum, Brug., to which frequent reference will 

 be made.* 



In nearly all the littoral species referred to, if a small portion 

 of the lamellae is taken, a circulation can be seen for some time 

 after the death of the animal. In Patella tra?noserica, Martyn, 

 the gill forms a fringe of separate cylindrical filaments round the 

 foot. There is no plate, properly speaking, though each separate 

 filament is a row of rods arranged at right angles or obliquely to 

 the length of the filament. This is a very peculiar structure, and 

 deserves attentive examination. Like all the gill-organs these 

 filaments are richly furnished with cilia. Down each side there 

 is a wide branchial artery, and the chitinous rods pass from one 

 to the other. Between these the blood can be seen circulating, 

 and the cilia in constant movement, causing the fluid to move in 

 two distinct currents along the narrow channels, so that the 

 corpuscles are visible passing in opposite directions or jostling one 

 another as they are hurried to and fro by the action of the cilia. 

 I have seen this action going on vigorously four hours after the 

 death of the animal. The whole surface of the filament, it should 

 be noticed, is studded with minute pores, possibly to permit a 

 more perfect oxygenation of the blood. The pores are apparently 

 smaller than the corpuscles, which are irregular in size and shape, 

 some being many times larger than the pores referred to. The 

 chitinous rods seem to be hollow tubes, darker in the centre from 

 granular cells. The rod continues to the end of the filament, 

 where the rounded ends give to the latter a wrinkled appearance. 



A somewhat different structure exists in the case of Chiton, 

 where the general plan of the branchise is the same ; that is, 

 continued round the foot. It is remarkable that there should be 

 such an extensive gill in the case of these genera, while in the 

 genus Acmcea, which is a conical shell attaining to nearly the same 

 size, a small gill-plume at the back of the neck comprises the 



* Adams in his notice of this animal, in the ' ' Voyage of the Samarang/' 

 gives a dissection of Cerithium, but avoids all mention of the gills. 



