1871.] WOODWAKD XTPHOSTIRA.. 59 



ledge extends, I must adhere to my already published conclusion, 

 namely that one pair of antennary organs is suppressed — a strong 

 argument in favour of the larval character of these palaeozoic forms. 



8, With regard to the under lip (metastoma), it seems singular 

 to base the refusal of a place to Limulus among the Crustacea upon 

 the fact that the metastoma is rudimentary, whereas in Pterygotus 

 it is very large, formed in one piece, having a ridge in the mesial 

 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 maxillge to retain the food in the same manner as 

 does the hypostome in Apus. Yet the Trilobita and Eurypte- 

 rida (one with a hypostome, and the other with a metastome) are 

 placed by Dr. Dohrn together, beside the Crustacea. The develop- 

 ment of this very organ in larval Limulus was one of the points 

 Dr. Dohrn was unable to clear up. 



9. With regard to Dr. Dohrn's application of Darwin's theory 

 of evolution, it cannot be necessary in this place to set forth the 

 many ways in which this doctrine has been found most serviceable 

 to' the naturalist, as offering the means of solution to many a 

 " Gordian knot " — sometimes untying it for us with the greatest 

 ease — at other times (and possibly in this case), cutting it with 

 equal facility. 



It is no difficult matter to puU 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 evi- 

 dences for maintaining the existing arrangement may be care- 

 fully reconsidered. 



If we are contented to conclude with Dr. Dohrn that " the mor- 

 phologico -genealogical relations of these three families of Crustacea 



(" telson "), but is composed of several of the most posterior abdominal segments 

 welded together. 



He further confirms my statement, already recorded in my former paper, 

 that the thoracic or opercular plate derives its innervation from a cephalic gan- 

 glion, whereas the inner and posterior branchiiferous plates are appendages of 

 the tlioracico-abdominal somites. 



