ARTHROPODA OF DOUBTFUL POSITION. 



87 



terminated by a bayonet-shaped or spoon-shaped caudal spine. No abdominal limbs 

 are present. JEurypterus remipes, the typical species, is common to the Silurian rocks 

 of Europe and America. Eusarcus scorpionis occurs in the water-lhne group of 

 Buffalo, N. Y. Pterygotus is another prominent genus. It is supposed that the 

 Eurypterida swam near the surface, and were more predatory animals than the king- 

 crabs. 



In the lower carboniferous rocks of Great Britain and the con- 

 tinent a number of small fossils have been found, the systematic posi- 

 tion of which is extremely doubtful. From their approximately 

 circular outline they have received the generic name Cyclus. In 

 length they average between a quarter and a third of an inch. Dr. de 

 Koninck would consider them as most nearly related to the trilobites, 

 while Dr. Woodward, the able paleontologist of the British Museum, 

 thinks that they may be " the larval stages of JPresttvichia, Belinurus, 

 etc., the ante-types in carboniferous times of the modern king-crab." 



FIG. 120. Cyclus 

 jonesianus, en- 

 larged. . 



FlG. 121. Linguatula diesinr/ii, enlarged four times. 



PENTASTOMIDA. 



In the Pentastomida, for which no common name exists, we have a type so modi- 

 fied by a parasitic life that it is difficult to trace at first sight even arthropodan char- 

 acters, much less features by which it 

 can be assigned to any of the well- 

 defined groups, although it seems 

 probable that, like the Tardigrada, it 

 belongs near the mites. The body is 

 long and worm-like, and is constricted 

 by numerous thickenings of the body-wall, so that it appears as if made up of many 

 short segments. The mouth occurs near the anterior end of the body, and on either 

 side of it are two very minute curved hooks, and in front of them a pair of rudi- 

 mentary palpiform organs. These hooks can be drawn into small openings, and as 

 these with the mouth are five in number, the applicability of the name (five mouths) 

 is evident. Each of these hooks is solid, and is moved by appropriate muscles inside 

 of the body. Another name, Linguatulina (little tongue), has been given to these 

 forms. 



From the mouth the alimentary tract pursues a nearly straight course the whole 

 length of the body, being held in place by a membrane similar to the mesentery of the 

 vertebrates. The nervous system consists of a ring around the oesophagus, with a pos- 

 terior enlargement, from which nerves are given off to the different portions of the 

 body. The sexes are distinct, the males being usually much smaller than the females. 

 The eggs undergo their development in the ovary, and the larva? resemble, in general 

 appearance, both the young of the mites and of some of the parasitic Crustacea. 



The habits of these forms are more interesting than is their structure, and in this 

 respect they show a great resemblance to the cestoid worms, of which an account is 

 given in the first volume of this series. In their sexless condition they are found in 

 the lungs and liver of various rodents and herbivorous mammals, and also of some 

 of the reptiles. Leuckart has traced out the life-history of some of the forms with 

 great care. When the flesh of the hare or rabbit containing these forms is consumed 



