THE PAXTOPODA. 



187 



part is a large ganglior.ic mass, forming a ring that embraces the oesophagus and gives off stems 

 to various parts of the body ; and no trace of respiratory organs can be detected, although, in some 

 cases, there appear to be apertures in the skin representing stigmata. 



From the researches of Van Beneden and Schubarth it appears clear that the Linguatulina 

 undergo a retrograde metamorphosis, and that while in the young state they show a distinct alliance 

 with tiie lowest Arachnids, in their adult form they present strong resemblances to the parasitic worms, 

 and it is curious that, like the latter, they have to migrate from one animal to another in order to 

 attain their sexual maturity. The best known species (Pentastoma tcenioides) is found when adult in, 

 the frontal and nasal cavities of the Dog and Wolf, where it doubtless causes much irritation. The 

 eggs, when laid, are discharged with the mucus from the nose, fall upon plants, and are eaten 

 with them by Hares and ether mammals. The embryos hatched from these eggs, which show 

 Arthropod characters, bore their way from the 

 stomach into the liver, where they enclose 

 themselves in a capsule, and change their skin 

 several times. In about half a year they acquire 

 the worm-like form, and again begin a migration, 

 piercing through the liver, an operation which 

 is sometimes fatal to their unfortunate host. 

 Otherwise they again become encapsuled, and 

 remain in this state until the animal containing 

 them is devoured by some dog-like animal, when 

 they at once make their way into the air-cavities, 

 and wait there for their sexual maturity. This 

 is the ascertained history of Pentastoma twnioides, 

 and it is believed that the other species of the 

 group have the same habits. The number of 

 known species is about twenty, but more have 

 been recorded owing to the individuals encysted 

 in the liver or lungs of herbivorous animals 

 being taken for distinct species. They form the 

 single family Acanthotheca. 



ORDER VI. PANTOPODA. 



This order includes a small number of curious 

 marine animals which have been regarded by 

 different authors either as Arachnida or Crus- 

 tacea, and really seem in some respects intermediate between the two classes. They have a cepha- 

 lothorax of four distinct segments, which constitutes nearly the whole of the body, the abdomen 

 being represented only by a small rudimentary part seen between the bases of the hind pair 

 of legs. No respiratory organs can be recognised, but a three-chambered heart has been detected in 

 them. The legs, of which there are four pairs (the first pair representing 

 palpi), are long and many-jointed, and in general they bear such a proportion 

 to the body that the name Pantopoda (All-legs) applied to the order is 

 peculiarly appropriate. The order has also been called " Podosomata," 

 which expresses very nearly the same idea. The mouth is suctorial and 

 forms a sort of rostrum, projecting in front of the cephalothorax. At the 

 base of this, on the back of the cephalothorax, four eyes are placed on a 

 tubercle. 



In the interior the alimentary canal runs straight through the body, 

 but the narrow stomach gives off on each side five blind tubes, which not 

 only run towards the limbs as in the Arachnida generally, but actually 

 a-averse nearly the whole length of the legs, whilst the fifth (short) pair run up into the chelicerse. 

 The sexual organs are situated in a very singular position in both sexes, namely, in the fourth or 



AMMOTHOA PYCNOGONOIDES. 

 ce, rostrum ; an, chelicei se ; v, stomach ; i, intestine ; p, p, blind tubes. 



LARVA OF NYMPHOX. 



