CHARACTERS OP THE SUB-KINGDOMS. 19 



are sometimes confined to one region of the body, and 

 sometimes distributed over all the segments of it. 



The blood is a colourless fluid, containing corpuscles. 

 The blood-vessels are situated above the alimentary canal. 



Respiration is performed differently in the various 

 classes. 



The principal class in the division Anarthropoda is the 

 Annelida or worms, which are destitute of jointed limbs. 



SUB-KINGDOM III. Annuloida. 



16. The members of this sub-kingdom were formerly 

 included in the Annulosa, but have now been separated 

 from it and erected into an independent group. The 

 reasons assigned by Professor Huxley for this separation 

 are : Their bodies are not marked by definite rings or 

 segments, there is no longitudinal chain of ganglia, and 

 none of them possess limbs arranged in pairs. 



Two classes have been assigned to this primary division, 



1. Echinodermata (Gr. echinos, a hedgehog; derma, 



skin), including sea-urchins, star-fishes, &c. ; and, 



2. Scolecida (Gr. scolex, a worm), consisting princi- 



pally of internal parasites. 



At first sight there seems to be very little resemblance 

 between these two classes, a sea-urchin and a tape-worm 

 being as unlike one another as possible. They both, 

 however, possess what has been called the "water- vascular" 

 system, that is, "a series of canals communicating with 

 the exterior by means of apertures situated upon the 

 surface of the body, and branching out more or less 

 extensively into its substance." 



SUB-KINGDOM IV. Mollusca. 



17. The Mollusca (Lat. mottis, soft,) are invertebrated 

 animals whose bodies are covered by a soft skin, which 

 envelops them like a loose sac. This covering is called 

 the mantle. It secretes the shell which most of them 

 possess, and to it the muscles are attached. This shell is 



