188 Arthropoda. 



different Arthropods presents very considerable variations, which will 

 be dealt with later; in a few forms (Acarines, small Crustacea), it 

 is entirely wanting. The blood is usually a colourless fluid, with 

 colourless, amoeboid blood corpuscles. 



From some small Crustacea respiratory organs are entirely 

 absent ; generally, however, there are either gills, or peculiar air- 

 breathing organs ((See special classes). 



Excretory organs. The segmental organs, familiar 

 in the Chsetopods, occur again in one division of the Arthropods, but 

 reduced to a small number, two pairs ; the antennary and shell- 

 glands of Crustacea (see this group) are modified segmental 

 organs. In Insecta, Myriapoda, and Arachnida there is, on the other 

 hand, no trace of such structures ; instead, they possess the so-called 

 Malpighian tubes, long glandular canals, which open into the 

 hind gut and perform an excretory function. 



Genitalia. The Arthropoda are, with few exceptions, of 

 separate sexes: the male and the female glands closely resemble 

 one another. There is never more than one pair of genital glands, 

 and this is frequently united or even fused to form an unpaired 

 organ. From each gonad springs a duct (the oviduct or vas deferens), 

 which opens on the ventral side, always in front of the anus ; the 

 ducts are frequently united for the last part of their course, and then 

 there is only one aperture. Even when the glands are connected or 

 fused there are generally two ducts. 



Class 1. C^stacea. 



The head is never sharply marked off from the rest of the body 

 (as is the case among Insecta, for example), but some of the thoracic 

 segments are usually fused with it. It bears, besides the eyes, which 

 will be dealt with later, two pairs of antennae (the antennulee 

 and the antennse), and three pairs of jaws, the mandibles and 

 the first and second maxillee. The antennse are usually elon- 

 gate, whip-like appendages, consisting of a short, jointed, basal piece, 

 or peduncle, and a long, flexible end-piece, composed of many 

 joints ; or the peduncle may bear two such filaments. The most 

 important part of the mandibles is a hard, unsegmented, basal 

 piece, the true mandible, which is provided, as a rule, on the inner 

 side with a sharp dentate edge, and often with a rough, grinding 

 surface. The sharp edge, as well as the grinding surface, works against 

 the corresponding parts of the other mandible. The basal part often 

 bears a smaller jointed appendage, a "palp." The other pairs of 

 jaws are not nearly as strong as the mandibles : they are lamellate, 

 and the inner edge is divided into several lobes, beset with stiff setse ; 



