Miscellaneo us. 127 



itself, the foot gradually is less and less used as an organ of locomo- 

 tion, until it entirely disappears in Ostrea. The retractor muscles of 

 the foot, now practically useless organs, are, however, still present. 

 The loss of one adductor muscle can probably be referred to 

 mechanical causes. In studying the change of relation of the ful- 

 crum to the adductors, he found that as the fulcrum moved forward 

 (anteriorly) it increased the distance from the posterior, aud lessened 

 the distance from the anterior muscle. As this took place, the 

 muscle furthest from the fulcrum was always the larger ; in fact it 

 must of necessity be so, as more power was needed at this point, 

 while the near one, from the fact that it does not require much 

 power, diminishes in size. In Plana one muscle is very much, in 

 fact four or five times, larger than the other, the smaller being 

 close to the apex of the shell, in other words, close to the fulcrum. 



As the fulcrum passes still further forward, a point is soon i-eached 

 when both muscles come in Hue with the fulcrum ; the larger one 

 in this case takes all the work from the smaller one, which, from its 

 now useless position, degenerates to disappearance. 



A procedure from regular to irregular shell is to be seen in the 

 freshwater forms. Uaio, he held, is probably a freshwater Mt/tllus, 

 which does not have any byssus present in the adult, but has one in 

 the embryo. A form that closely resembles the oyster can be traced 

 through ^Etheria to Muelleria, the so-called freshwater oyster. The 

 latter has both adductors in the embryo, but only one, like Ostrea, 

 in the adult. 



Passing now in the other direction. Dr. Sharp pointed out the 

 stages connecting the central type to the extreme in AspergiUum. 



In passing out from the central type, the Areas, the group known 

 as the Siphonata appear, where, besides the large foot, it is found 

 that the aboral portion of the mantle has united at two or three 

 points, forming one or two tubes. In some forms of Lucina, 

 by the union of the mantle a single tube is formed, the so-called 

 anal siphon, which corresponds to the superior one when two are 

 present ; through this passes the water outwards, the inflowing 

 water passing in through the large space between the mantle-edges, 

 as in the asiphonated forms. In this form of Lucina specialization 

 has only determined the direction of the outflowing current, which 

 carries off the deoxygenated water and the excreta. 



In Cardium the siphon is made up of two tubes ; in other words 

 the ingoing and outgoing currents are now determined. The edges 

 of the mantle begin to adhere, leaving room only for the pro- 

 trusion of the foot. In Venus the arrangement is practically the 

 same : — a well-developed siphon, large wedge-like foot, which is a 

 locomotor organ, a shell entirely covering the animal when it is 

 closed, and two well-developed adductors, equal in size. The 

 specialization in this line of development is in the direction of the 

 siphon and closure of the mantle. Mt/a would represent a form 

 leading to Solen ; here the siphon is large, the mantle more or less 



