436 THE MICROSCOPE. 



but nothing more. In the higher forms, the body may be 

 said to be divided into two parts, the anterior of which, 

 as in the Crustacea, consists of the thoracic segments, 

 amalgamated with those of the head, and forming together 

 a mass called the cephalothorax. In the highest the 

 division of the thorax into separate segments becomes 

 apparent ; but the anterior segment is still amalgamated 

 with the head. The structure of the abdomen varies 

 greatly. In some cases it forms a soft round mass, with- 

 out any traces of segmentation; whilst in others, as 

 scorpions, it is continued into a long flexible jointed 

 tail 



The Annulosa is divided, by Professor Huxley, into two 

 principal groups, the Articulata and the Annuloida. The 

 Articulata, 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 

 sensation 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 Annuloida; nevertheless, there are two features in 

 which they all agree ; firstly, they possess a remarkable 

 system of vessels, either ciliated, or deprived of cilia, 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 

 structure 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 



