78 NATURAL HISTORY OF ARTHROPODS. 



AKTHKOPODA OF DOUBTFUL POSITION. 



As was mentioned on a preceding page, there are several groups of jointed-footed 

 animals the position of which, in the branch of Arthropoda, is very uncertain. No 

 one has yet conclusively proved that any of these forms belong to either the crustaceans 

 or to the insects, and it may possibly be ascertained that some at least are to be con- 

 sidered as equivalents to the groups just mentioned. With this great uncertainty we 

 have considered it advisable to place these forms in an intermediate position between 

 the Insects and Crustacea, and in their treatment to omit the terms class, sub-class, 

 and order, except in one group where it seems that their use is warranted. With the 

 exception of Limulus, the living forms are generally small and not of frequent occur- 

 rence, facts which in part explain the uncertainty which exists regarding their 

 systematic position. 



PYONOGONIDA. 



The " Sea Spiders " form a problematical group which agrees with the true 

 Arachnida in the possession of eight pairs of legs, but they present so many points of 



difference from them, and indeed from all Tra- 

 cheata, as to justify Dr. Hoek, their latest student, 

 in regarding them as a class. The body is always 

 small, and is divided into four segments, the an- 

 terior of which has received the name of cephalo- 

 thorax. To this is attached a proboscis, which 

 has recently been shown to be not the united 

 chelicerae and pedipalps, as was supposed by Pro- 

 fessor Huxley. Behind the proboscis are three 

 pairs of appendages, the mandibles, palpi, a-nd 

 ovigerous legs. Sometimes the first two pairs 

 are lacking in the adults, but the ovigerous legs 



~ O 



are always present, and only one exception is 

 FIG. 107. PhoxicMUdium maxiiiare, sea-spider, known to the rule that the male bears the effffs 



twice natural size. 



attached to these limbs. On the doi'sal surface 



of the cephalothorax is a small prominence known as the oculiferous tubercle, which 

 bears the four simple eyes. These eyes, however, are occasionally rudimentary or 

 absent, especially in the deep-sea forms. 



The thoracic portion of the cephalothorax, and the three thoracic segments, bear 

 four pairs of walking feet, each supported on an outgrowth from the body. The legs 

 are long, frequently many times the length of the body, and are eight-jointed. The last 

 thoracic segment also bears the rudimentary abdomen, which is occasionally articulated. 



The nervous system consists of a brain and five (rarely four) infra-oesophageal 

 ganglia, connected by commissures. From the brain nerves go to the eyes, mandibles, 

 and upper portion of the proboscis ; from the first ganglion behind the oesophagus, the 

 lower portion of the proboscis, the palpi, and ovigerous legs are innervated. The other 

 ganglia supply the legs. The digestive system presents some interesting peculiarities. 



