TYPE CRUSTACEA. 



433 



probably serving for swimming. The abdomen was com- 

 posed of twelve segments, the anterior six of which were 

 much more massive than the others and bore five pairs of 

 platelike appendages on whose posterior surface were the 

 branchiae. The terminal segment bore a spine or fiulike 

 structure. Such a form as this, represented by the genus 

 Eurypterus (Fig. 198), presents strong similarities to Limulus 

 and also to the Scorpions, bearing out 

 the numerous similarities of structure 

 occurring between Limulus and those 

 forms. This side of the affinity may 

 be postponed, however, until the 

 next chapter, and the comparison of 

 Limulus with the Crustacea discussed 

 here. Its chitiuous cuticle, its jointed 

 and biramous appendages, and its 

 branchial respiration show similari- 

 ties to the Crustacea, as do also the 

 form of the heart and the compound 

 eyes. Whether or not the coxal 

 gland is comparable to the shell- 

 gland is at present uncertain, but the 

 other similarities are sufficient to 

 justify the recognition of a Crusta- 

 cean origin for Limulus. It forms FIG. 198 Eurypterus remipes 

 indeed a connecting link between the (from ^HOLSON). 



Crustacea and the Arachnida, presenting probably on the 

 whole more affinities with this latter group than with the 

 former. 



Since, however, a Crustacean ancestry is probable, a com- 

 parisiou of the appendages of Limulus with those of a repre- 

 sentative of the ancestral group ought to be possible. It has 

 already been noticed that the brain of Limulus is a syncere- 

 brum composed of three segments ; it represents, therefore, 

 two segments of which the appendages and other parts have 

 disappeared. Furthermore, recalling that, in the higher 

 Crustacea at least, a ganglion occurring between the cerebral 

 autennary ganglia in the embryo indicates a lost pair of 



