2 A HISTORY OF RECENT CRUSTACEA 



wormlike genus called Balanoglossus ; (4) the Vermes or 

 Worms; (5) the Artkropoda ; (6) the Mollusca, among which 

 are the well-known oyster, snail, and cuttle-fish ; (7) the 

 Molluscoidea, containing the mollusc-like lantern-shells, 

 and the grouped animals of the Polyzoa, in some of which 

 the so-called ' bird's-head ' organs amuse the observer ; 

 (8) the Tunicata, the tunic-clad or mantled animals, com- 

 prising the Ascidians, whether tough-coated or gelatinous, 

 and the Salpte which roam the sea in alternate genera- 

 tions solitary or connected in a chain ; (9) the Vertebrata, 

 with 'the important classes of fish, amphibians, reptiles, 

 birds, and mammals. 



It is with the central group of these nine that we are 

 here concerned. So far as the name goes the Arthropoda 

 are animals with jointed limbs. So far as the name goes, 

 therefore, cats and dogs and vertebrates in general might 

 belong to this division. But the name was given with 

 reference not to the vertebrates, but to the vermes, for 

 originally the worms and arthropods were included in a 

 division called the Annulosa, animals of which the bodies 

 contain several annuli, rings, metameres, somites, zonites, 

 arthromeres, or segments, as they are variously called. 

 These two sections of the Annulosa are now severed, and 

 are distinguished by the circumstance that the one is, and 

 the other is not, provided with jointed limbs.' 



The Arthropoda are defined as animals which have 

 bodies composed of variously shaped segments ; which 

 have jointed appendages attached to some at least of the 

 segments ; which have (in general) a brain united to a 

 ventral nerve-cord, or ganglionic chain, and which exhibit 

 bilateral symmetry. 



None of the other divisions will be found to possess all 

 these characters combined. For example, in the verte- 

 brata the nerve-cord is dorsal, in the mollusca the body is 

 unsegmented, in the vermes there are no jointed appen- 

 dages. Instances, it is true, are to be met with of arthro- 

 pods which do not themselves answer the requirements of 

 the definition, instances in which the body is unsymme- 

 trical or unsegmented, and in which there are no articu- 



