PHYSIOLOGY OF THE INVERTEBRATA. 47 



(13) The P('?^<rts^o??m^a is the first order of the Arachnida 

 — one of the five divisions of the Arthropoda. The only genus 

 is the parasitic Pentastomum. In the sexual state, the vermi- 

 form Fcntastomum is found in the nasal cavities of the Car- 

 nivora, while in the sexless condition it infests the liver and 

 lungs of the Herhivora and Rcptilia. There are ambulatory 

 limbs, the only appendages are four hooks, two on each side 

 of the mouth or oral aperture. The alimentary canal is very 

 simple, at the anterior end of which is a slender oesophagus 

 and at the posterior end the anus. 



(14) The Aretisca or Tardigrada. — Like the previously 

 mentioned order, the Aretisca are low-organised members of 

 the Arachnida ; yet there is an advance not only in the 

 alimentary canal, which is more in keeping with the highly 

 developed digestive system of the lasccta, but in the presence 

 of rudimentary legs. 



The genus Macrobiotics is found in moss and in sand. The 

 mouth (Fig. 9) is suctorial, which is situated at the end of a 

 rostrum, and is provided with two styles. It leads into a 

 shoi't oesophagus which passes into a muscular pharynx. The 

 ducts from two well-defined salivary glands discharge their 

 contents into the posterior part of the oral cavity. The 

 pharynx leads into a wide alimentary canal which gradually 

 narrows to the anus. 



(15) The Fi/cnogoiiida. — The alimentary canal of these 

 marine animals is very different from that of the two 

 previously mentioned orders. The oesophagus leads into a 

 more or less circular stomach which sends off very long 

 diverticula or ca3ca into the legs. There is a short rectum 

 terminating in an anus. 



(16) The Acarina. — This order includes the mites and 

 ticks. The alimentary canal is short and straight ; but the 

 stomach is produced into several c^cal appendages. Salivary 

 glands are sometimes present. In Ixodes the salivary glands 

 are situated on the sides of the anterior part of the body, 

 and pour the secretion into the mouth at the base of the 



