106 OUTLINES OF NATURAL HISTORY. 



posed of two distinct layers, an outer and an inner, 

 and the latter of these includes a distinct cavity, which 

 IS properly called a " body-cavity." There is a distinct 

 mouth, which opens either directly into the body-cavity, 

 or into a distinct stomach. The stomach, however, 

 when present, opens below into the body-cavity, so 

 that the latter in all cases communicates directly with 

 the outer world through the mouth. The integument is 

 provided with the singular microscopic organs of offence 

 and defence which are known as " thread-cells " or 

 " nettle-cells." There is no distinct blood-system or heart ; 

 and the nervous system is either absent or quite rudi- 

 mentary. The body does not exhibit any distinct seg- 

 mentation, but some of its parts are always arranged in a 

 more or less star-like or " radiate " manner. Examples of 

 the sub-kingdom are the Fresh-water Polypes {Hydra), 

 the Sea-anemones {Actinia), the Sea-firs, Sea - jellies. 

 Corals, &c. 



* StJB-KINGDOM TIL ANNULOIDA. 



The name Anmdoida (Latin, anmdus, a ring ; Greek, 

 eidos, form) is given to this sub-kingdom because of the 

 resemblance of some of the animals which it comprises to 

 the true Anmdosa or Kinged animals, such as the Worms. 

 This sub-kingdom includes the two classes of the Echino- 

 dermata and Scolecida, and is characterised by the follow- 

 ing peculiarities : — The body-cavity is completely shut off 

 from the digestive cavity (when this is present), so that 

 the body-cavity in no case communicates with the outer 

 world through the mouth. There is a distinct nervous 

 system, and sometimes a blood-system. In all, there is a 

 pecuKar system of "water-vessels," consisting of canals 

 which are distributed through the body, and in most cases 

 receive water from the outside through an aperture in the 

 walls of the body. The fully-grown animal is sometimes 

 more or less star-like or " radiate " in the arrangement of 

 its parts, sometimes more or less elongated and worm-like. 

 In no case, however, does the body consist of a regular 



