534 THE INVERTEBRATA 



ticks. They are, variously, scavengers, ectoparasites on all sorts of 

 plants and "hangers on" of all sorts of animals, but in the last case 

 they become, by the modification of the chelicerae and pedipalps, 

 blood-sucking parasites. 



In the most free-living of them, like the aquatic and predatory 

 Hydrachnidae, the chelicerae are clawed piercing weapons and the 

 pedipalps leg-like with sensory hairs. The chelate condition of the 

 chelicerae may be seen in the cheese mite, Tyroglyphus (Fig. 366), 

 which is a typical saprophyte living on cheese only when it has begun 

 to decay. The pedipalps are here no longer leg-like. 



In a tick Hke Argas (Figs. 365 A, 367, 368), the pedipalps are sensory, 

 but the chelicerae and the median hypostome are elongated and con- 

 verted into serrated cutting tools ; a sucking channel is formed between 

 these. The mouth is usually minute and leads into a sucking pharynx 

 and then into an endodermal stomach which gives rise to caeca in the 

 ticks, where there are also salivary glands of large size opening into 



Fig. 365. Dorsal surface of A, Argas (Argasidae) and B, Amhlyomma, $ 

 (Ixodidae). From Nuttall and Warburton. The Argasidae are distinguished 

 by the leathery skin, diversified by discs which mark the insertion of muscles ; 

 the Ixodidae by the hard scutum, which covers the whole body of the male 

 and the anterior part of the body in the female (B). 



the pharynx. The saliva is said to contain an anticoagulin, as in leeches, 

 and this renders easier the gradual digestion of the blood which is 

 taken into the stomach. A remarkable phenomenon without parallel 

 in the Arthropoda is the occurrence of intracellular digestion in some 

 acarines. The cells of the stomach put out pseudopodia and the blood 

 plasma is taken into vacuoles where it is digested. 



The circulation is extremely degenerate. No heart has been ob- 

 served with certainty and the blood system is lacunar in mites, but 

 in the tick, Argas, there is a single-chambered pulsating vessel with 

 a pair of ostia and an aorta running forward to a periganglionic sinus. 

 The respiratory organs are tracheae, long and convoluted. These open 



