cvi Introduction. 



or both pairs of maxillae are more or less completely fused mesially 

 so as to form a functional lower lip ; and^ with the exception of 

 the lower Myriopoda, their post-abdominal segments^ when present, 

 rarely or never carry appendages with locomotor functions in the 

 adult state. The portion of the supra-oesophag-eal ganglionic mass 

 which corresponds with the eyes, is much larger relatively to that 

 w^hich corresponds with the antennary organs than it is in Crus- 

 tacea; and in exact opposition to what we observe in this latter 

 class, we find the salivary and renal organs largely developed, and 

 the hepatic only represented rudimentarily. The digestive tract is 

 never aborted in air-breathing Arthropoda, nor aproctous in adult 

 individuals. Excej^t in certain lower Crustacea and Arachnida, 

 where the supra-oesophageal nerve-mass is represented simply by 

 a fibrous commissure, the nerve-system consists of supra-oesophageal 

 and of ventrally-placed ganglia, connected with each other so as 

 to form a collar round the oesophagus, and connected with a 

 sympathetic system ordinarily consisting of a ' stomatogastric ' 

 division and of ' nervi transversi,' 



All Arthropoda, with the exception of the Cirripedia and Tardi- 

 grada, are dioecious, and, with the exception of the Tetradecapodous 

 or Hedriophthalmatous Crustacea, and cornuted Insecta, the males 

 are ordinarily smaller in size than the females. Reproduction is 

 ordinarily sexual, but both parthenogenesis and asexual genesis by 

 means of pseudovaria, if not also by internal metagenesis without 

 the intermediation of such structures, are known to occur in this 

 Sub-kingdom. The Arthropoda are ordinarily oviparous, but are 

 sometimes viviparous, and even pupiparous. Except in the cases 

 of certain of the lower Crustacea and Arachnida, the segmentation 

 of the yolk is always partial, and the first appearance of the embryo 

 takes the shape of a ' primitive streak.' Though the embryos of 

 Arthropoda very ordinarily go through more or less numerous stages 

 of metamorphosis, neither larvae nor adults ever possess cdia. In 

 the more usually observable forms of metamorphosis, the embryo 

 leaves the egg not only with its reproductive system, but also with 

 its motor and sensory organs in a less perfect condition than those 

 of the adult ; in the less ordinary, or retrogressive metamorphosis, 

 observable in parasitic families, the animal organs of the larva are 

 more perfect than those of the adult. In both cases, change of 

 tegument accompanies metamorphosis. 



