530 E. RAY LANKESTER. 



Class 3. Crustacea. 



Grade a. — Eutomostraca. 



Ex. — Apus, Braucliipus, C^^clops, Balanus. 

 Grade h. Malacostraca. 



Ex. — Neb alia, Astacus, Oniscus, Gam- 

 mar us. 

 Class 4. Chilopoda. 



Ex. — Scolopendra. 

 Class 5. Hexapoda (syii. In sect a Pterygota). 



Ex, — Locusta, Phryganea, Papilio, Apis, 

 Muse a, Cimex, Lucanus, Macliilis. 

 IncertiB sedis. — Tardigrada, Pentastomida3 (degeuerate 

 forms). 



The Segmentation of tlie 13ody of Arthropod a. — 

 The body of the Arthropoda is more or less clearly divided 

 into a series of rings, segments, or somites, which can be 

 shown to be repetitions one of another, possessing identical 

 parts and organs which may be larger or smaller, modified 

 in shape or altogether suppressed in one somite as compared 

 with another. A similar constitution of the body is more 

 clearly seen in the Chastopod worms. In the Vertebrata also 

 a repetition of units of structure (myotomes, vertebra?, etc.) — 

 which is essentially of the same nature as the repetition in 

 Arthropods and Ch^topods, but in many respects subject to 

 peculiar developments — is observed. The name ''meta- 

 merism " has been given to this structural phenomenon be- 

 cause the " meres," or repeated units, follow one another in 

 line. Each such " mere " is often called a " metamere." 

 Tliis is not the place in Avhich to discuss the origin and 

 essential nature of " metamerism " or " metameric segmenta- 

 tion.'^ Nevertheless a satisfactory consideration of the 

 structure of the Arthropoda demands a knowledge of what 

 may be called the laws of metamerism. These are not so 

 fully ascertained or formulated as might be expected. The 

 repetition of parts, Avhich we note as metamerism, is, as 

 Haeckel, Batesou, and others have recognised, only a special 



