ZOOLOGICAL POSITION OF LIMULUS. 225 



line. In the Trilobita it is wanting, and in place of it we have a largely developed 

 hypostome, or upper lip, fulfilling the function of assisting the maxillae to retain the food 

 in the same manner as does the hypostome in Apus. Yet the Trilobita and Eurypterida 

 (one with a hypostome, and the other with a metastome) are placed by Dr. Dohrn 

 together in one section, beside the Crustacea. The development of this very organ in 

 larval Limulus was one of the points which Dr. Dohrn was unable to clear up. 



As to Dr. Dohrn's proposal to remove the Merostomata and the Trilobita from 

 the Crustacea and place them in a separate class beside the Crustacea, and to include with 

 them the Arachnida, I have made elsewhere 1 the following remarks : 



"It is no difficult matter to pull down a system of classification ; but it is only right 

 to demand that this should not be done upon insufficient grounds. 



" Heretofore, in the formation of zoological groups, it had been the custom (long before 

 embryology became so important a branch of study, or was so well understood as it is 

 at this day) to take the sum of all the characters which the species presented, giving due 

 value and weight to each ; and this method has been adhered to in the classification both 

 of the animal and the vegetable kingdom by all our leading naturalists. 



" The introduction of embryological investigations has furnished an additional and 

 conclusive support in most cases to the results of the exhaustive method of examination 

 already applied to the adult form, seldom aiding us so much in differentiating group 

 from group as in pointing out affinities, and thereby inviting us to throw down boundaries 

 (hitherto scrupulously guarded by the systematic naturalist) and to merge together larger 

 and yet larger groups. 



" Before this is done all I would beg is, that the facts and evidences for maintaining 

 the existing arrangement may be carefully reconsidered. 



" If we are contented to conclude with Dr. Dohrn that ' the morphologico-genealogical 

 relations of these three families of Crustacea (viz. Limulida, Eurypterida, and Trilobita) 

 cannot at present be established, perhaps may always remain doubtful,' and that we 

 should therefore combine them under one collective name, ' Gigantostraca,' placing 

 them ' beside ' the Crustacea, we lay ourselves open to the grave charge of destroying an 

 established system without " offering any good ground for " setting up a new one in its 

 stead. 



" Take away the Trilobita from the pedigree of the Crustacea, and I submit that one of 

 the main arguments in favour of evolution to be derived from the class, so far from being 

 strengthened, is destroyed. Erom what are the Crustacea of to-day derived ? Are we 

 to assume that they are all descended from the Phyllopods and Ostracods — the only two 

 remaining orders whose life-history is conterminous with that of the Trilobita ? Or are we 

 to assume that the Arachnida are the older class ? 



" ' If/ as Eritz Muller well observes, ' all the classes of the Arthropoda (Crustacea, 

 Insecta, Myriopoda, and Arachnida) are indeed all branches of a common stem (and of this 

 1 ' Quart. Journ. Geol. Soc.,' 1871, vol. xxviii, p. 59. 



