INVER TEBRA TES. 7 



and pearly nautilus. They may be placed highest among 

 Invertebrates since many of them exhibit a concentration 

 of the nervous system greater than occurs elsewhere. 



Unlike Vertebrates, and such Invertebrates as Insects and 

 Crustaceans, Molluscs are without segments and without 

 appendages. A muscular protrusion of the ventral surface, 

 known as the " foot," serves in the majority as an organ of 

 locomotion. In most cases, a single or double fold of 

 -skin, called the "mantle," makes a protective shell. The 

 nervous system has three chief pairs of nerve centres or 

 ganglia. In many cases, the larval stages are very char- 

 acteristic. 



Arthropods. 



This large series includes Crustaceans, Myriopods, Insects, 

 Spiders, and other forms, which have segmented bilaterally 

 symmetrical bodies and jointed appendages. The skin 

 produces an external cuticle, the organic part of which con- 

 sists of a substance called chitin, associated in Crustaceans 

 with carbonate of lime. The nervous system consists of a 

 dorsal brain, connected, by a nerve ring around the gullet, 

 with a ventral chain of ganglia. 



Echinoderms. 



This is a well-defined series, including starfishes, brittle 

 stars, sea urchins, sea cucumbers, and feather stars. The 

 r symmetry of the adult is usually radial, though that of the 

 i larva is bilateral. A peculiar system, known as the water-; 

 vascular system, is characteristic, and is turned to various 

 uses, as in locomotion and respiration. There is a marked 

 tendency to deposition of lime in the tissues. The develop- 

 ment is strangely circuitous or " indirect." 



Segmented " Worms" 



It is hopeless at present to arrange with any definiteness 

 those heterogeneous forms to which the title " worm " is 

 given. For this title is little more than a name for a shape, 

 assumed by animals of varied nature who began to move 

 head foremost and to acquire sides. There is no class of 

 " worms," but an assemblage a mob not yet reduced to 

 order. It seems useful, however, to separate those which 



