344 Professor Huxley [May 11, 



analogy is permissible on this subject, it is probable that the sensi- 

 bility of the oyster is infinitesimally small. 



The oyster, as we have seen, possesses one very large adductor 

 muscle, but only one. Almost all other Lamellibranchs (e. g. cockles, 

 mussels, razor-fish) have two ; one in front, near the mouth ; and one 

 behind, in a position which exactly answers to that of the single 

 adductor of the oyster. The latter, therefore, is called monomyary, 

 or one-muscled, while the former are dimyary, or two-muscled ; and a 

 series of forms can be selected among the sea mussels and the scallops 

 which show the posterior adductor becoming larger and larger, while 

 the anterior diminishes, until, in the oyster, it disappears. 



During the summer and autumn months, from as early as May to 

 as late as, or even later than, September, according to circumstances, 

 of which the temperature and the depth of the water in which the 

 oysters live appear to be the most influential, a certain proportion of 

 the oysters in an oyster-bed pass into a peculiar condition, and are 

 said by the fishermen to be " sick." In about half of these sick 

 oysters, a whitish substance made up of innumerable very minute gra- 

 nules, imbedded in, and held together by, a sort of slime, collects in 

 the infra-branchial chamber, filling up the interspaces between the 

 mouth and the gills, and between the gill plates themselves, and even 

 occupying the vestibular cavity so completely that it is difficult to 

 understand how the processes of breathing and feeding can be carried 

 on (Fig. 2 (B) ). 



This granular slime is what is known as " white spat," and the 

 granules are the eggs of the oyster. By degrees, the granules become 

 more or less coloured; and the mass, acquiring a brownish hue, is 

 termed " black spat." This change depends on the development 

 of the young, which acquire a certain degree of coloration within the 

 eggs. At the end of a period, the length of which varies with the 

 temperature of the water and other conditions, but appears rarely to 

 exceed a fortnight, the mass of black spat breaks up, and the young, 

 hatched out of the eggs, leave the mantle cavity of the parent in which 

 they have been thus incubated. They become difi'used through the 

 water, and swarm in vast multitudes at the surface of the sea. 



A single full-grown oyster produces, on the average, about a 

 million of these free-swimming young or larvcE. If a glass vessel 

 is filled from the stratum of surface-water in which the larvae swim, 

 and held up to the light, it will appear full of minute particles — 

 only y^^oth of an inch long, and therefore just visible to the naked 

 eye — which are in active motion. An ordinary hand-magnifier is 

 sufficient to show that these minute organisms have very much the 

 aspect of the Botifera, or " wheel animalcules," so common in fresh 

 water. They have a glassy transparency, and are colourless, except 

 for one or more dark brown patches ; while, at one end, there is a 

 disk, like the " wheel " of the Rotifers, the margins of which are 

 apparently in rapid motion, and which serve as the organs of pro- 



