ANNUL0SA. 559 



the bivalve shell of a mussel ; others are simply folded at 

 the back, so as to appear like a bivalve, but are really 

 not so ; or they consist of a number of rings or seg- 

 ments. The body of the Cypris presents a reticulated 

 appearance, resembling that of cell structure. All the 

 Entomostraca are best preserved in a solution of chloride 

 of calcium. 



Annulosa. — Articulata. The animals composing the 

 sub-kingdom Articulata are characterised by having the 

 body enclosed in a tunic, or integument, consisting of a 

 series of rings, segments, or joints, " articulated " together 

 by a flexible membrane. 



The Annulosa are divided, by Professor Huxley, into 

 two principal groups, the Arthropoda and the Annuloida. 

 The Arthropoda, comprising Insecta, Myriapoda, Crustacea, 

 and Arachnida, possess a definitely segmented body ; the 

 segments being provided with appendages, the anterior of 

 which are so modified as to subserve the functions of sen- 

 sation and manducation. They have almost always a heart, 

 communicating with the general cavity of the body, for 

 propelling the true corpusculated blood which that cavity 

 contains. The nervous system consists of a longer or a 

 shorter chain of ganglia. 



Nothing can be more variable than the characters of the 

 body, the appendages, and the nervous system among the 

 rest of the Annulosa, which are included under the Annu- 

 loida; nevertheless, there are two features in which they 

 all agree ; firstly, they possess a remarkable system of 

 vessels, either ciliated, or deprived of cdia, and containing 

 a fluid very different from the true blood which fills the 

 general cavity of the body or perivisceral space ; secondly, 

 in no annuloid animal has any true heart been hitherto 

 discovered. Contractile vessels belonging to the system 

 just referred to abound, but no organ comparable in struc- 

 ture to the heart of other animals has yet been found in 

 any of the Annuloida. 



The Annuloida, as thus defined and limited, fall into 

 two parallel series ; in one of which, for the most part, 

 dioecious forms predominate, as the Annelida, while of 

 the latter, the Trematoda may be regarded as the typical 

 example ; on the other hand, the Echinodermata and 



