14 



INTRODUCTORY LECTURE. 



corresponding symmetrical form. The skeleton is external, and 

 consists of articulated segments, of frequently an annular form : the 

 articulated limbs in those species which possess them have a similar 

 condition of the hard parts, in the form of a sheath, which encloses 

 the muscles. 



The respiratory organs commonly open upon the sides of the body; 

 rarely near the anus, and never communicate with the mouth. 



The jaws, when present, are lateral, and move from without 

 inwards, and not from above downwards. 



The heart is situated in the back, is often vasiform ; and the veins 

 are frequently in the form of large, irregular sinuses ; there is always 

 a circulation, and the blood is red in one class (Anellides). 

 Most Articulata are dioecious : a few are hermaphrodites. 

 The Radiata, or fourth primary division of animals in the system 

 of Cuvier, is so called because most of the species comprising it have 

 their parts arranged around an axis, on one or several radii, or on 

 one or several lines extending from one pole to the other. The 

 nervous system, when traces of it have been visible, is also arranged 



in radii {Jig. 4.). It does not 

 present the Homogangliate or 

 Heterogangliate type. In one 

 family only {Holothuriadce) is 

 there a distinct respiratory sys- 

 tem : the other characters as- 

 signed by Cuvier are negative 

 ones. 



I have already observed, that 

 there is no instance in which 

 animals, grouped together by 

 negative characters, have formed 

 a natural assemblage ; nor is the 

 sub-kingdom Hadiata of Cuvier 

 an exception to this rule. 

 The truth is simply that the anatomy of this immense assemblage 

 of low-organised animals is not yet sufficiently understood; and, con- 

 sequently, general propositions, and at the same time positive ones, 

 like those which define the Vertebrate, Molluscous, and Articulate 

 sub-kingdoms, cannot be enunciated. 



Much has unquestionably been done in this field of Natural 

 History since the time of Cuvier, and attempts have been made, with 

 various degrees of success, to subdivide the Radiata according to 

 positive characters. 



