Brooks.] 232 [February 2, 



although the fact that Anodonta has an unmistakable velum would 

 seem to indicate that the Lamellibranchs, like the Gasteropods, are 

 the descendants of a free-swimming veliger, and that the circlet of 

 cilia described in the embryos of such forms as Cardium is also to be 

 regarded as a rudiment of the same stage. It may be that the devel- 

 opment of the young within the branchiae or the mantle chamber in 

 this class does away with the necessity for a locomotive embryo, but 

 at present we know so little of the life history of the marine forms 

 that we have very little ground for generalization. The imperfection 

 of our present knowledge cannot, however, be fairly urged to re- 

 strain us from making as much use as possible of what knowledge we 

 do possess, although we must constantly bear in mind that it intro- 

 duces an element of uncertainty into all of our conclusions. This, 

 of course, is true of all biological speculation at present, but no one 

 would advocate the abandonment of all speculation and comparison 

 until all of the facts of our science have been recorded and verified. 



The embryo of Anodonta, at a very early stage, has, at the ante- 

 rior end of the worm-like body, a simple band of cilia; as develop- 

 ment progresses this is carried, by the formation of the mantle lobes, 

 into the mantle cavity, and there increases in length, and the free 

 ends bend towards each other and finally unite, thus forming a closed, 

 bilaterally lobed circlet like that of the Gasteropods, except that it 

 is not raised from the surface of the body, and its cilia are very short 

 and are not used for locomotion. It is interesting to notice also that 

 it is attached to the dorsal surface of the shell by two muscles like 

 those of the veliger of a Gasteropod. In Anodonta these subsequently 

 become the retractor muscles of the foot. 



The thecosomatous Pteropoda present the veliger stage of develop- 

 ment in a form as highly specialized as that of the marine Gastero- 

 poda, and the embryos of the two do not differ at this time in any 

 essential particular. The development of the gymnosomatous Pter- 

 opods on the contrary is entirely anomalous, and at present appears to 

 be inexplicable on any theory of descent. 



In the Cephalopoda, as so often happens in the higher representa- 

 tives of a group, the indirect course of development has given place 

 to the direct; the larval stages are usually entirely wanting, and the 

 embryo shapes itself, from the beginning, into the form of the adult. 

 In most Cephalopods there is no trace of a veliger stage, but its ab- 

 sence is what we should expect from the analogy of the higher forms 

 of other groups. 



