xi PHYLUM ARTHROPODA 659 



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 pairs of short legs. 



THE TARDIGRADA. 



The Tardigrada ("Bear-animalcules") are soft-skinned animals (Fig. 570) 

 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 surrounded by papillae ; the buccal cavity contains 

 a pair of horny, sometimes partly calcified, teeth (styl.}. The ducts of a pair 

 of salivary (?) glands (soli) 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 respiration, and heart and blood-vessels are likewise 

 absent. There are 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. Segmentation 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. 



RELATIONSHIPS OF THE AIR-BREATHING ARTHROPODA. 1 



Notwithstanding the existence of some striking superficial 

 resemblances between the Arachnida and the Insecta, the evidence 

 afforded by anatomy and embryology points to the conclusion that 

 there is no direct genetic relationship between the two groups. 

 The occurrence in both of a peculiar form of respiratory organs, 

 the tracheae, seems at first sight to indicate such a relationship ; 

 but the evidence of an independent origin is so strong that it 

 must be supposed that the tracheae have been independently 

 developed in the two classes. The most important points of 

 difference are the separation of head and thorax in the Insecta, 

 the mode of development of the eyes, the presence in the Arachnida 

 of an extensive " liver," and (perhaps) the endodermal origin of 

 the Malpighian tubes in the latter class. 



Resemblances between Limulus and the Scorpions are readily 

 apparent. In both there is a cephalothorax bearing six pairs of 

 appendages, together with two median and several lateral eyes. 

 The appendages in both are all originally post-oral, the first 



1 The Xiphosura, and also the Pentastomida, though not air-breathing, are 

 discussed here. 



