184 ARTHROPODA. 



or phosphate of lime. A heart is found in most forms ; it 

 is placed dorsally, and is provided with paired slits, termed 

 ostia. The body-cavity contains blood. In some forms 

 respiration takes place by means of the general surface of 

 the body ; others are provided with special organs gills 

 (or branchiae), tracheae, pulmonary sacs (or lung-books). 

 The gills are generally processes given off from some of 

 the appendages; the tracheae are branching tubes filled 

 with air and opening to the exterior ; the pulmonary sacs 

 are infoldings of the integument. The nervous system 

 consists of a supra-oasophageal ganglion, a ring round the 

 oesophagus, and a ventral cord, usually with ganglia, placed 

 beneath the intestine. Cilia are absent in all members of 

 the sub-kingdom, with the exception of Peripatus. The 

 sexes are separate in almost all forms. 



The Arthropoda are divided into the following classes, 

 (1) Crustacea, (2) Prototracheata, (3) Myriapoda, (4) In- 

 secta, (5) Arachnida. The last four breathe by means of 

 tracheae or pulmonary sacs, and are therefore often grouped 

 together as the Tracheata, the Crustacea which breathe 

 by means of gills, forming the Branchiata. The Tracheate 

 forms are comparatively rare as fossils, and consequently 

 no description of them will be given here : the Myriapoda 

 include the millipedes and centipedes, the oldest form is 

 from the Old Red Sandstone; the Insects occur first in 

 the Upper Silurian, they become more abundant in the 

 Coal Measures, and are fairly common in some of the 

 Mesozoic and Cainozoic deposits, as for instance the Lias, 

 the Solenhofen Slates, the Purbeck, and the Bembridge 

 Beds ; the Arachnida include amongst others the scorpions 

 and spiders, the earliest scorpion is found in the Silurian, 

 the earliest spider in the Coal Measures. 



