Invertebrate Gallery of the Indian Museum. 53 



latter phylum, namely the Rotifera and Cha^topoda, have 

 been classed with the Arthropoda by Professor Ray Lan- 

 kester as a snigle phylum under the name Appendicu- 

 lata ; the two phyla resemble each other in the prime 

 fact that they are alike made up of a repetition of rings 

 or segments (somites), but whereas in the Annelida the 

 segments, with the exception of the two which form the 

 head and the one which forms the posterior limit, are all 

 very nearly alike and equivalent in function and structure, 

 in most of the Arthropoda the segments differ in function, 

 and consequently in structure, in the different regions of 

 the body. Instead of a body constructed of a succession 

 of nearly similar rings or segments, we now find in the adult 

 Arthropod, a body in which the segments usually fall into 

 three dissimilar groups or regions — a head region, a thorax 

 region, and an abdominal region — and even within the limits 

 of any one region, but especially in the regions of the 

 head and thorax, the constituent segments may differ from 

 each other. Another important morphological difference 

 between the Annelida and the Arthropoda is that in the 

 Annelida the segmental (locomotor) appendages, when they 

 exist, are unjointed, whereas in the Arthropoda these ap- 

 pendages, or limbs, are built up of a succession of different 

 pieces all movably jointed together — from which fact the 

 name Arthropoda is derived, viz., "joint-footed" animals, 

 or animals with jointed legs. A third less important and 

 less constant difference, which relates to the manner of 

 life, is that the Annelida usually have soft bodies, while the 

 body and limbs of the adult Arthropod are generally 

 encased in an armour of chitin. 



The head region of the Arthropod consists of several 

 segments which are often fused together, their appendages 

 being modified to form sensory organs (or antennae), and 

 biting and chewing organs (or mandibles and maxillae). 



The thorax region, which lies behind the head and is 

 often fused with it to form a cephalothorax, also consists 

 of several intimately fused segments, the appendages of 



