Zoological Classification. 369 



parable to the heart of a crustacean or an insect ; but a system 

 of vessels, with more or less extensively contractile walls, con- 

 taining a clear fluid, usually red or green in colour, and, in some 

 rare cases only, corpusculated, is very generally developed, and 

 sends prolongations into the respiratory organs when such 

 exist/' Professor Huxley likewise states that "the embryos of 

 annelids are very generally ciliated, and vibratile cilia are 

 commonly if not universally developed in some part or other 

 of their organization.-" " In both these respects," he adds, 

 "they present a most marked contrast to the succeeding 

 classes. - " 



In the Crustacea the body is distinguishable into a variable 

 number of " somites " or definite segments, each of which 

 may be, and some of which always are, provided with a single 

 pair of articulated appendages ... a pair of ganglia is primi- 

 tively developed in each somite," and " no trace of a water- 

 vascular system, nor of any vascular system similar to that of 

 the Annelida, is to be found in any true Crustacean." 



With considerable resemblance to some Crustaceans, the 

 Arachnida — scorpions, mites, and ticks — never possess more 

 than four pairs of locomotive limbs, and the somites of the 

 abdomen are not provided with limbs. The Myriapoda, or 

 centipedes, have more than twenty somites in their bodies, and 

 , cf those which correspond with the abdomen of Arachnida 

 are provided with locomotive limbs." The Insecta have 

 tracheal respiratory organs like the Myriapoda, " with a ner- 

 vous and circulatory system disposed essentially as in this and 

 the two preceding classes ; but the total number of somites of 

 the body never exceeds twenty ;" and in the adult stage, the 

 abdomen is never furnished with locomotive limbs. 



The vertebrate animals consist of five classes : Fishes, 

 Amphibia, Reptiles, Birds, and Mammals. In a diagrammatic* 

 representation of a cross section of the higher invertebrate 

 animals, they are depicted as single tubes, in which are con- 

 tained the alimentary system, occupying the centre, the ner- 

 vous system above it, and the heart, or vascular system, 

 below. In Professor Huxley's words, "the external or inte- 

 gumentary and parietal portion of the blastoderm f never be- 

 comes developed into more than a single saccular or tubular 

 investment, which encloses all the viscera ; . . . . but in 

 the five vertebrate classes, the parietal portion of the blasto- 

 derm of the embryo always becomes raised up, upon each 

 side of the middle line, as a ridge, so that a long groove is 



* An anatomical or physiological diagram illustrates a principle of construc- 

 tion. It is not a likeness of any one thing. 



t Literally " germinal skin or membrane," the covering of the matter which 

 grows into an embryo, or of the embryo itself. 



