1 64 MORPHOLOGY OF INVERTEBRATE TYPES 



pendages is very hard. The articulation membranes are on 

 the other hand quite soft and elastic. The integument of the 

 abdomen is considerably thicker than that of the intersegmental 

 membranes, but is still elastic as is clearly demonstrated by 

 the considerable stretching of the abdomen under the pressure 

 of the growing ovaries. The entire body, excepting the articu- 

 lation membranes, is covered with hair. This is of at least two 

 types. One is long, serrated, but not branched; the other is 

 shorter, plumose. Every hair is the outgrowth of a single hypo- 

 dermal cell and is supplied with a nerve fibre. Thus the whole 

 surface of the body is very sensitive to touch or to air-currents 

 and vibrations. The spines as well as the claws and the spinning 

 tubes are also modified hairs. The spines sit in special sockets 

 and may be considerably lifted from their normal position in 

 which they practically lie on the surface of the limb. A new 

 cuticle with all its hair is produced underneath the old one and 

 takes the place of the latter after each moulting. 



Endoskeleton. An endoskeleton is present in both 

 cephalothorax and abdomen. In the latter it consists of three 

 rudimentary endosternites serving for the attachment of muscles. 

 The endoskeleton of the cephalothorax is of a very peculiar 

 shape. It consists of a central, saddle-like plate situated between 

 the thoracic ganglionic mass and the ring of intestinal coeca, 

 and of a number of processes. Some of these processes serve 

 for the attachment of muscles which hold the endoskeleton as 

 it were suspended horizontally, others for the attachment of 

 muscles inserted in the coxae of the limbs. 



Muscular system. The muscular system is naturally 

 very complicated and highly specialized. It still retains traces 

 of metamerization in the abdomen and thorax. Of special in- 

 terest are the muscles of the pumping stomach. The single 

 dorsal one is attached to the fold of the cephalic groove, the 

 two ventral muscles to processes of the endoskeleton. Very 

 powerful are the muscles moving the chelicera. The muscular 

 system of the legs and pedipalpi is characterized by the absence 



