14 



INTRODDCTORY LECTURE. 



The jaws, when present, are lateral, and move from side to side. 

 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: still 

 fewer are parthenogenetic. 



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 Radiata 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. 



The binary division, which I proposed in 1835 f, was founded on 

 the following considerations. The Radiata of Cuvier, in which the 

 nervous system could be most unequivocally traced in a filamentary 

 form, present an alimentary canal as a distinct tube, with a mouth 



* First demonstrated by Hunter, in the Crustaceans and Insects. Sec " Treatise 

 on the Blood," p. 174., and X. vol. ii. p. 138. pi. xvii. 



t Syllabus of the Lectures on Comparative Anatomy, given at the Medical 

 School of St. Bartholomew's. 8vo. 



