XII] 



XIPHOSURA 



275 



lies inside the body near the ventral surface. It is formed of modi- 

 fied connective tissue in which a cheesy material termed chondrin 

 has been deposited in the ground substance, and it is largely built up 

 of interlacing tendons of muscles so that it acts as an internal support- 

 ing structure or endoskeleton. It is called the endosternite. 

 Possibly it was a feature of primitive Arthropoda, as similar 

 endosternites occur in many other Arachnida and in some of the 

 more primitive Crustacea. 



The internal anatomy differs in many points of detail from that 

 of the Spider, but in essentials there is a fairly close resemblance. 

 Unlike the Scorpion, Limulus lays eggs and these are fertilised 

 in the water and pass through what may be termed a larval stage. 



FIG. 126. Section through the operculum and gills of a King-crab, Limulus. 

 x about 16. The normal number of gills in a Limulus is five, the section 

 from which this drawing is made showed only four. 



1. Operculum. 2. 2nd gill-book. 3. Muscle which moves gills up and 

 down. 4. Blood-vessels. 5. Muscle which raises the operculum. 



In many respects Limulus seems to be related to the extinct 

 Eurypterina, whose fossil forms are so abundant in the Upper 

 Silurian and Old Red Sandstone formations ; and like some species 

 of Limulus they attained a great size, two feet or more in length 

 being not uncommon. The Eurypterines were aquatic and indeed 

 seem to form an intermediate stage between the Scorpion and 

 Limulus, and confirm us in the conclusion drawn from the anatomy 

 of Limulus that this animal retains in many points the habits and 

 structure of the marine ancestors of Arachnida. 



The PANTOPODA, familiarly known as Sea-Spiders, are small 

 marine animals about the size of spiders or harvest-men, which 

 are found under the stones at low tides on many parts of our 

 coast. They live on colonies of Hydrozoa and other small 

 sessile animals. They resemble Arachnida in some points and 



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