ANNULOIDA : ECHINODERMATA. 121 



ANNULOIDA. 



CHAPTER XVIII. 



1. GENERAL CHARACTEES OF THE ANNULOIDA. 2. GENERAL 

 CHARACTERS OF THE ECHINODERMATA. 



SUB-KINGDOM III. ANNULOIDA (= Echinozoa, Allman). This 

 sub-kingdom was proposed by Professor Huxley for the re- 

 ception of the two groups of the Echinodermata and the Sco- 

 lecida, of which the former belonged to the old sub-kingdom 

 liadiata ; whilst the latter was formerly classed with the 

 Annulosa. The same sections have been grouped by Professor 

 Allman together, under the name Echinozoa ; the Rotifera, 

 however, being excluded from this division and classed with 

 the Annulosa. By others, again, the Annuloida are looked 

 upon as a section of the Annulosa, and not as a distinct sub- 

 kingdom. Provisionally, however, it seems best to regard 

 the Annuloida as one of the primary divisions of the animal 

 kingdom, it being impossible, in the meanwhile, to frame a 

 definition common to it and to the Annulosa. 



The Annuloida are distinguished by the presence of a distinct 

 nervous system, and the possession of an alimentary canal which is 

 entirely shut off from the general cavity of the body. A peculiar 

 system of canals, usually communicating with the exterior, and 

 termed the ' water-vascular, 1 or l aquiferous,' system is present 

 in all ; and a true vascular apparatus is sometimes present. In 

 none is the body of the adult composed of definite segments, or 

 provided with ' bilaterally disposed successive pairs of ap- 

 pendages.' 



By Professor Allman the following definition has been given 

 of the Annuloida or Echinozoa : 



'Animals composed of numerous merosomes' (body-seg- 

 ments) ' which are disposed radially round a longitudinal 

 axis; always with a determinable, median, antero-posterior 

 and dorso-ventral plane. A distinct body-cavity from which 

 the alimentary canal is entirely shut off, and which, therefore, 

 never communicates with the outer world through the 

 mouth.' 



The Annuloida are divided into two great classes, the 

 Bchinodermata and the Scolecida. 



