""""" LIFE-HISTORY OF THE OYSTER. 321 



ered from the preceding account that these organs are quite different 

 in structure from those of fishes, in which we find two rows of branchial 

 processes, arranged lilje the teeth of a comb, supported at the outer 

 margin of about five bony arches, with gill clefts or slits between the 

 latter, which open outwards from the fore part of the sides of the throat. 

 In the oyster the gills have no connection with the throat, and have 

 moreover, as already stated, the form of elongated sacks, with porous 

 walls, with a row of large pores opening above into the cloaca cJ, as 

 shown in Fig. 1. The lateral pores between the ribs on the gills, and 

 opening into the cavity of the latter, and these cavities in turn opening 

 by way of the rows of large pores, 6p, into the cloaca cl, permit the 

 water necessary for respiration to readily pass through the gills, as indi- 

 cated by the course of the arrows in Fig. 1. 



The way in which the water is forced through them is, however, 

 quite different from that observed in fishes, in which the water is 

 pumped through the gills by the action of the mouth and gill-covers. 

 In the oyster, on the other hand, fresh supplies of water are swept 

 through the pores and internal cavities of the gills in an entirely differ- 

 ent way, viz, by means of very numerous minute and slender processes 

 with which these organs are covered. These processes, or cilia, as they 

 are properly called, vibrate or swing to and fro many times per second, 

 and more forcibly in one direction than in another, so that they set 

 up a current of water in the direction of their most forcible vibration. 

 This, in brief, is tlie means by which the water is swept through the 

 gills of the oyster in a continuous stream, ministering to respiration or 

 oxygenation of the blood of the animal in its passage through the 

 branchial organs. 



The blood of the oyster is normally colorless, and much more watery 

 than in higher animals with red blood, in which the blood-cells or cor- 

 puscles are also discoidal or oval and flattened, while in the oyster they 

 are nearly globular, as usually seen floating in the serum. They meas- 

 ure about one three-thousandth of an inch in diameter, but vary some- 

 what in size. They are in reality very small lumps of protoplasmic 

 matter, provided with a nucleus embedded in their substance in an 

 eccentric position. They undergo great changes of form when taken 

 from the animal alive, and may live for four hours under the micro- 

 scope, during which time they may be observed to slowly tlirust out 

 finger-like portions of their substance in various directions, and even 

 move about slowly by means of a progressive flowing motion of their 

 own glairy substance, much like those remarkably simple animals found 

 in ponds and ditches, and known to naturalists under the name of am- 

 cebce. In their movements, as watched under the microscope, two or 

 more blood-cells of the oyster may even actually flow together and be- 

 come confluent. Their function is in all probability of very much the 

 same nature as that of the analogous corpuscles found in the vessels of 

 higher animals, viz, to minister to respiration and the processes of 

 3 INT 21 



