CIRCULATORY SYSTEM OF THE GIANT SCALLOP. 24! 



We may possibly conceive that the ancestor of the Mollusca 

 was among the early ones to recognize the advantages of the 

 ocean bottom, and that its race soon developed a protective shell, 

 if this had not started to form before it became a dweller on the 

 bottom. The shell would offer protection, but would, because of 

 weight, interfere with rapid movement. As enemies became able 

 to get beneath its armor the shell became thickened and was 

 made to cover the animal more completely, but the added weight 

 interfered still more with rapid movement. 



At this time we need not suppose that the animal had more 

 than the very simplest nervous system, hardly more than that 

 needed by a trocophore larva, for it would probably be depend- 

 ent upon simple bands of cilia, or at the most a movable mouth 

 portion, for getting its food. There is no reason for supposing 

 that this animal had yet developed gills, or if gills were present 

 they would hardly be more than simple folds of the mantle. 



As competition became more severe, animals of this kind were 

 in need of better protection, and it is possible to conceive that 

 there might have been evolved two types, one that inclosed itself 

 in a bivalve shell, crawled into the mud, and obtained its food by 

 capturing the forms brought to it in a current of water of its own 

 creation, the other, more like the hypothetical primitive niulliisk 

 that IMS been described, which retained a single shell and got its 

 food by creeping over the bottom and picking it up directly. 

 The first form would still have a simple head apparatus and 

 would need new nervous centers only to provide for the mech- 

 anism necessary to crawl into the mud and the mechanisms 

 necessary to create the current of water and capture the living 

 forms from it. The second form would have a more complicated 

 head apparatus and would need nervous centers to supply it and 

 to supply the organ by means of which it was enabled to creep. 

 In these differences in life, and in the consequent differences in 

 structure, it seems reasonable to look for the differences in their 

 nervous systems. If this conception is anything like true, from 

 very early times there was no similarity in the method these two 

 groups used in getting food. One has finally developed a re- 

 markably satisfactory method of straining out living particles that 

 serve it as food, from a current of water of its own formation, and 



