MOLLUSKS 



WE now come to another large and important group of ani- 

 mals, with characters so peculiar to itself and differing so 

 radically from those which distinguish any other class of living 

 creatures, that it may be said to occupy almost a unique position 

 in the animal kingdom. Recent investigations have demonstrated 

 that the larval form of mollusks presents some remarkable 

 points of similarity to the embryonic forms of the Annelida 

 and to the larvae of some other classes of lower organisms. 

 This discovery is one of the triumphs of embryology in its patient 

 search for that connecting-thread that weaves together all the 

 varying forms of animal life. Aside from these subtle evidences 

 of relationship revealed by the microscope, the mollusks appear 

 to occupy a position of considerable isolation in the biological 

 world. 



As accepted by zoologists to-day, this phylum is but the rem- 

 nant of its former self. Aristotle considered all creatures with 

 a testaceous covering to belong to a single family, and those 

 later patriarchs of biology, Linnaeus, Cuvier, and Lamarck, ex- 

 tended the group to include the greater part of all the marine 

 invertebrate animals. Little by little the phylum has been shorn 

 of orders and classes. First, the worms and the EcMnodermata 

 were separated into distinct phyla ; then the barnacles were dis- 

 covered to be crustaceans, and were accordingly removed from 

 their position as " multivalves " under the Mollusca ; then the 

 tunicates, or ascidians, were found, through the critical examina- 

 tion of their larval stage, to be merely masquerading as mollusks ; 

 and lastly, the brachiopods have been somewhat reluctantly re- 

 moved from their old position with the mollusks and given the 

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