STRUCTURE OF INTEGUMENT 669 



through the ostia into the chamber of the heart and is propelled forwards 

 along the aorta and lateral vessels. 



In proportion to the size of the animal the nervous system of the tick 

 is very small. It consists of a single ganglionic mass (Plate LXXXIII, 



fig. 1), representing the fused ganglia and termed the 



, . ~, - ,. 7r , The Nervous System 



brain. This structure lies in the triangular space 



anterior to the mid-intestine, superficial to the vagina in the female 

 and to the ' white gland ' in the male. It is traversed by the oesophagus 

 in the usual manner. Branches are given off directly for the supply of the 

 legs and the internal organs. On section it is seen to consist of an outer 

 zone of nerve cells and an inner non-medullated zone. 



Ticks do not possess a large amount of connective tissue ; there are, 

 however, a number of loose strands connected by tracheae, which form 

 a cellular matrix to support the organs; in stained speci- 

 mens small nuclei may be seen lying on the strands. 

 In the nymph and the adult there are in addition 

 many large single cells, especially marked just below the skin and along 

 the main tracheae, which appear to be true fat body cells. In the egg- 

 depositing female these increase in size and later undergo a fatty change, 

 their protoplasm becoming filled with clear droplets ; in the Ixodini they 

 are very conspicuous and may be mistaken for parasites. 



The integument of a tick, is of a leathery character. In the Ixodini it 

 is formed of a semi-transparent, smooth layer ofchitin which is hardened 



in certain parts to form rigid plates and shields, such 



. . The Integument 



as the scutum, anal plates, etc. In the Argatim, on the 



other hand, the integument may be either adorned with discs which are 

 arranged in radiating lines and which serve for 'the attachment of 

 muscles, or it may have mammillae giving it a granular appearance ; 

 there are no shields or plates of hardened chitin. On section the skin 

 is seen to be composed of two layers, an outer chitinous and an inner 

 cellular layer. The chitinous layer varies in thickness, and in the unfed 

 condition is indented by furrows which give it a serrated appearance 

 on section ; the external portion, therefore, appears to be more rigid 

 than the internal, which is elastic and does not become folded. In 

 the full-fed tick the chitinous layer is almost smooth and is thicker 

 than in the starving condition. The cellular layer is formed of cells which 

 vary in shape according to the condition of the animal. In the unfed 

 state they are columnar in shape, while in the fed tick they are cubical. 

 Their protoplasm is finely granular and stains very readily with haema- 

 toxylin ; the nuclei are situated near the basal end and are round or oval 



