ITS ZOOLOGICAL POSITION. 11 



the larger tendons, internal ridges and partitions, and the lining 

 membrane of extensive internal cavities, such as the alimentary 

 canal, and the air-tubes of Insects. 



In most Arthropoda the body is provided with many appen- 

 dages. In Crustacea there are often twenty pairs, but some 

 Myriopoda have not far from two hundred pairs. Some of 

 these may be converted to very peculiar functions ; in particular, 

 several pairs adjacent to the mouth are usually appropriated to 

 mastication. One or more pairs of appendages are often trans- 

 formed into antennae. 



The relative position of the chief organs of the body, viz. : 

 heart, nerve-cord, and alimentary canal, is constant in Arthro- 

 poda. The heart is dorsal, the nerve-cord ventral, the 

 alimentary canal intermediate. (See fig. 3.) The oesophagus 

 passes between the connectives of the nerve-cord. Not a few 

 other animals, such as Annelids and Mollusca, exhibit the same 

 arrangement. 



Arthropoda are not known to be ciliated in any part of the 

 body, or in any stage of growth. Another histological pecu- 

 liarity, not quite so universal, is the striation of the muscular 

 fibres throughout the bodv. In nianv Invertebrates there are 



o / / 



no striated muscles at all, while in Vertebrates only voluntary 

 muscles, as a rule, are striated. 



The circulatory organs of Arthropoda vary greatly in plan 

 and degree of complication, but there is never a completely 

 closed circulation. 



The development of Arthropoda may be accompanied by 

 striking metamorphosis, e.g., in many marine Crustacea, but, as 

 in other animals, the terrestrial and fluviatile forms usually 

 develop directly. Even in Insects, which appear to contradict 

 this rule flatly, the exception is more apparent than real. The 

 Insect emerges from the egg as a fully formed larva, and so far 

 its development is direct. It is the full-grown larva, however, 

 which corresponds most nearly to the adult Myriopod, while the 

 pupa and imago are stages peculiar to the Insect. It is not by 

 any process of embryonic development, but by a secondary 

 metamorphosis of the adult that the Insect acquires the power 

 of flight necessary for the deposit of eggs in a new site. 



