ARACHNIDA. 117 



ARACHNIDA. 



No attempt will be made to pass in review all of the subclasses of the arachnids. Some 

 of the Merostornata are so obviously trilobite-like that it would seem that their relationship 

 could easily be proved. The task has not yet been satisfactorily accomplished, however, 

 and new information seems only to add to the difficulties. 



So far as I know, the Aranese have not previously been compared directly with trilobites, 

 although such treatment consists merely in calling attention to their crustacean affinities, as 

 has often been done. 



Carpenter's excellent summary (1903, p. 347) of the relationship of the Arachnida to 

 the trilobites may well be quoted at this point : 



The discussion in a former section of this essay on the relationship between the various orders of 

 Arachnida led to the conclusion that the primitive arachnids were aquatic animals, breathing by means of 

 appendicular gills. Naturally, therefore, we compare the arachnids with the Crustacea rather than with the 

 Insecta. The immediate progenitors of the Arachnida appear to have possessed a head with four pairs of 

 limbs, a thorax with three segments, and an abdomen with thirteen segments and' a telson, only six of which 

 can be clearly shown by comparative morphology to have carried appendicular gills. But embryological 

 evidence enables us to postulate with confidence still more remote ancestors in which the head carried well 

 developed compound eyes and five pairs of appendages, while it may be supposed that all the abdominal 

 segments, except the anal, bore limbs. In these very ancient arthropods, all the limbs, except the feelers, 

 had ambulatory and branchial branches ; and one important feature in the evolution of the Arachnida must 

 have been the division of labour between the anterior and posterior limbs, the former becoming specialized 

 for locomotion, the latter for breathing. Another was the loss of feelers and the degeneration of the com- 

 pound eyes. Thus we are led to trace the Arachnida (including the Merostornata and Xiphosura) back to 

 ancestors which can not be regarded as arachnids, but which were identical with the primitive trilobites, and 

 near the ancestral stock of the whole crustacean class. 



TRILOBITES N9T ARACHNIDA. 



While no one having any real knowledge of the Trilobita has adopted Lankester's scheme 

 of the inclusion of the group as the primitive grade in the Arachnida, reference to it may 

 not be amiss. This theory is best set forth in the Encyclopaedia Britannica, Eleventh 

 Edition, under the article on Arachnida. It is there pointed out that the primitive arachnid, 

 like the primitive crustacean, should be an animal without a fixed number of somites, and 

 without definitely grouped tagmata. As Lankester words it, they should be anomomeristic 

 and anomotagmatic. The trilobites are such animals, and he considers them Arachnida and 

 not Crustacea for the following reasons : 



Firstly and chiefly, because they have only one pair (apart from the eyes) of pre-oral 

 appendages. "This fact renders their association with the Crustacea impossible, if classifi- 

 cation is to be the expression of genetic affinity inferred from structural coincidence." 



Secondly, the lateral eyes resemble no known eyes so closely as the lateral eyes of 

 Limulus. 



Thirdly, the trilobation of the head and body, due to the expansion and flattening of 

 the sides or pleura, is like that of Limulus, but "no crustacean exhibits this trilobite form." 



Fourthly, there is a tendency to form a pygidial or telsonic shield, "a fusion of the pos- 

 terior somites of the body, which is precisely identical in character with the metasomatic 

 carapace of Limulus." No crustacean shows metasomatic fusion of segments. 



Fifthly, a large post-anal spine is developed "in some trilobites" (he refers to a figure 

 of Dalinanitcs). 



