626 



ZOOLOGY 



SECT. 



in the Dog and Wolf. Its embryos escaping and falling on grass and other 

 herbage, which form the food of Hares and Rabbits, are taken up by the latter, 

 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 it should be received into the mouth of a. 

 Dog (still contained probably in most cases in the tissues of the Hare or Rabbit) 

 the young Pentastomum 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 pairs of short legs. 



fit. 



THE TARDIGRADA. 



The Tardigrada ("bear animalcules ") are soft-skinned animals (Fig. 522) 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 from the rest an anterior part or head. 

 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 papilla? ; the buccal cavity 

 contains a pair of horny, sometimes partly 

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

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

 cavity of the mouth ; there is a muscular 

 pharynx (ph.), a narrow resophagus, 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 o 

 Malpighian tubes. The muscles are al 

 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 eyespots situated at the anterior 

 end are the only representatives o 

 organs of special sense. The gonads in 

 both sexes are saccular, and open int( 

 the terminal part of the intestine. Seg 

 mentation is complete and regular. The 

 young animal at one stage has only twc 

 pairs of rudimentary legs, but develops 

 the full number before being hatched. 

 In the larva there is a head and four distinct segments. 



Some of the Tardigrada live amongst damp moss, others in fresh or in salt 

 water. 



FIG. 522. Macrobiotus hufelandi. 



I IV, appendages ; bucc. buccal cavity ; 

 !/l'/. accessory gland ; mal. Malpighian 

 tube ; or. ovary ; rect. rectum ; sali. 

 salivary glands ; atom, stomach ; styl. 

 teeth. (From Hertwig's Lehrbuch, 

 after Greef and Plate.) 



