PHYLUM ARTHROPOD A 



675 



Dog tul Wolf. Its embryos, escaping and falling on grass and other herbage, 

 which form the food of Hares and Rabbits, are taken up by these animals, and 

 perforating the wall of the alimentary canal, by means of a boring apparatus 

 composed of several chitinous pieces, lodge themselves in the liver, where they 

 become encysted and undergo a metamorphosis. Afterwards they leave the 

 cysts and move about. If the young Pentastomum should be received into the 

 mouth of a Dog (still contained probably in most cases in the tissues of the Hare 

 or Rabbit) it may find its way to the frontal sinuses or maxillary antra, there 

 to undergo its final transformation into the adult form. The larva possesses two 

 paii-s of short legs. 



The Tardigrada. 



The Tardigrada (" Bear-animalcules") are soft-skinned animals (Fig. 560) of 

 minute size, not exceeding a millimetre in length. The body is unsegmented and 

 not distinguishable into regions, except 

 that in some a slight constriction separates 

 off an anterior part or head from the rest. 

 The mouth is provided with a sucking 

 proboscis. There are four pairs of short 

 unjointed legs (I. — IV.), the last of which 

 is terminal, and each is provided with 

 two or four claws. The mouth is sur- 

 rounded by papillae ; the buccal cavity 

 contains a pair of horny, sometimes partly 

 calcified, teeth (styl.). The ducts of a pair 

 of salivary (?) glands (ncUi) open into the 

 cavity of the mouth ;■ there is a muscular 

 pharynx (ph.), a narrow oesophagus, and 

 a wide mesenteron (stom.) ; the anus is 

 sub-terminal, situated in front of the last 

 pair of limbs. A pair of tubes (mal.) 

 which open into the terminal part of the 

 intestine are perhaps representatives of 

 Malpighian tubes. The muscles are all 

 non-striated. There are no organs of re- 

 spiration, and heart and blood-vessels are 

 likewise absent. There is a brain and a 

 ventral nerve-cord with four ganglia. 

 Two eye- spots situated at the anterior 

 end are the only representatives of 

 organs of special sense. The gonads in 

 both sexes are saccular, and open into 

 the terminal part of the intestine. Seg- 

 mentation is complete and regular. The 

 young animal at one stage has only two 

 pairs of rudimentary legs, but develops the 



full number before being hatched. The larva possesses a head and four distinct 

 segments. 



Some of the Tardigrada live among damp moss ; others in fresh or in salt 

 water. 



Fig. 560.— Macrobiotus hufelandi. 



I — IV, appendages ; bucc. buccal cavity; 

 (//«/. accessory gland ; mal. Malpighian 

 tube ; or. ovary ; red. rectum ; soli. 

 salivary glands ; stout, stomach ; styl. 

 teeth. (From Hertwig's Lehrbuch, 

 after Greef and Plate.) 



