ZOOLOGICAL POSITION OP LIMULUS. 



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He further assumes that there are but five segments in the head of the Eurypterida, 

 each bearing a pair of appendages (and also two pairs of eyes, compound and simple), 

 but this head he regards as a cephalothorax comparable with that of Limulus, " the 

 thoracic segments being potential, but with one segment less, that bearing the first pair of 

 maxillipeds." Dr. Packard further argues that the head in Limulus contains organs 

 which in the Malacostraca are lodged in the thoracic region. To this we would reply 

 that in the Decapoda-Brachyura the carapace, which is formed (not by the thoracic 

 somites beneath it, but) by the coalescence of the anterior cephalic somites (antennal 

 and mandibular rings, Spence Bate), encloses precisely similar organs to those found 

 within the head-shield of Limulus, and moreover acts as a roof to the branchial chamber ; 

 the branchiae in the Crabs being attached to the thoracic feet as in the King-crabs, 

 although the feet themselves are so differently modified in the latter. 



5th. Dr. Packard has not understood Professor Huxley's explanation that " the 

 ovigerous segment " is considered by him as the last cephalic (or the first thoracic, 

 coalesced with the cephalon), and deriving its nerves from thence as shown by Professor 

 Owen (see ante, p. 196) ; hence arises a part of the difficulty which he finds in agreeing 

 with me as to the number of segments in Limulus and Pterygotus. The rest is occasioned 

 by attempting to show that the thoracic somites always remain undeveloped in Limulus and 

 Pterygotus, and that the head is really the cephalothorax, and the hinder body is the 

 abdomen, with an excessive number of segments as in Branc/tipus. At the risk of 

 seeming tedious we therefore venture once more to summarise our views of the 

 Merostomata. 



Eurypterida. 



I. Cephalon: — Bearing the 



C Ocelli and compound 

 (. Eyes on the dorsal surface. 

 First antennae } 1 pair 

 Second antennae ) aborted. 1 

 Mandibles. 



5. First maxillae. 



6. Second maxillae. 



7. Maxillipeds. 



1. 



2. 

 3. 

 4. 



XlPHOSUKA. 



Cephalon : — Bearing the 

 ( Ocelli and compound 

 (. Eyes on the dorsal surface. 



2. First antennae. 



3. Second antennae. 



4. Mandibles. 



5. First maxillae. 



G. Second maxillae. 

 7. Maxillipeds. 



1 Prof. Huxley has suggested that the chelate organs in front of the head in Pterygotus represent the 

 antennae, and that the antennules are aborted. If this holds good the remaining organs will be found to 

 correspond exactly in number with Limulus. In Pterygotus the basal joint of these first chelae are non- 

 mandibulate, in JEurypterus, Slimonia, and Stylonurus the first pair are simple palpi, but are mandibulate 

 at their base ; this would lead one to conclude that they probably represent the antennules in the first 

 and the antennae in the last-named species. * 



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