ON THE DEVELOPMENT OF LIMULUS LOXUISPIXA. 91 



Relation ofLimulus with Other Arthropods, with Some Remarks 

 on the Arthropod Ancestry oflhe Chordata. 



'i'he clo.^est rcJative uf Liiindus auioi-.g tlic Arthropüda is un- 

 douljtedJy tlie Trilobite.s. The relations l^etweeo them have already 

 been g'iven in the previous section. Tlunigh the appendages and the 

 internal structure of the Tril(.)l)itey are but little known, their external 

 features match I hose of no other animal 8o well as those i)f l/hiiulus. The 

 five-lobed céphalothorax, the three-lobed abdomen, and the eyes under 

 the facial suture, are the characteristic features of both Linuilus [unl the 

 'I'rilobites. Tliesc characterislics are not distinctly found in other 

 animals, even if not entirely wanting, ^[oreover, Liiiinlns and the Trilo- 

 bitetj resemble each other in their haljitat — swarming in the muddy 

 bottom of a shallow sea. 



The chief ditterences between Limnhis and the Ti'ilobites are as 

 follows: — In the Triloljites, some anterior abdominal segments ;»re 

 separate, while in Liiiiulus all the abdominal segments except the last 

 one are united into a single shield. In the Trilol)ites some posterior 

 al)dominnl segments are united and form the pygidium, while in 

 Liiindns the last ^-cgment is separate from all the other abdominal 

 aegments and forms the sword-like telson. In the Tiilobites the two 

 lateral lobes (jf the free cheeks are generally not uinted in the dorsal 

 median line, and the median eyes are generally absent*, while in 

 Liiiivhis the two lateral lobes of the free cheeks are united in front and 

 a pair of tlie median eyes is present. 



Those differences ai'c not of mucli importance and do not speak 

 against a close relationshi[) existing between the Trilobites and Liiindus. 



The next near allies of Liiitidas are the Merostomata Çlvury- 



* Present in Hurpc^. 



