230 THE CANADIAN NATURALIST. [Vol. vi. 



has demonstrated that Asaphus possessed eight pairs of five-jointed 

 legs of uniform size." 



Such statements are apt to mislead, unless we carefully compare 

 the characters of each group. And first let me express a caution 

 against the too hasty construction of a classification based upon 

 larval characters. 



Larval characters are useful guide-posts in defining great 

 groups, and also in indicating affinities between great groups ; 

 but the more we become acquainted with larval forms the greater 

 will be our tendency (if we attempt to base our classification on 

 their study) to merge groups together which we had before held 

 to be distinct. 



To take a familiar instance : if we compare the larval stages of 

 the common Shore-Crab (^Carcinus moenas) with Pteri/gotus, we 

 should be obliged (according to the arguments of Dr. Packard) 

 to place them near to or in the same group. 



The eyes in both are sessile, the functions of locomotion, 

 prehension^and mastication are all performed by one set of appen- 

 dages, which are attached to the mouth ; the abdominal segments 

 in both are natatory, but destitute of any appendages. 



Such characters, however, are common to the larvae of many 

 Crustaceans widely separated when adult, the fact being that in 

 the larval stage we find in this group, what has been so often 

 observed by naturalists in other groups of the animal kingdom, 

 namely, a shadowing forth in the larval stages. of the road along 

 which its ancestors travelled ere they arrived from the remote 

 past at the living present. 



If we place the characters o^ Limulus and Pterygotus, side by 



side, and also those of Trilobita and Isopoda, we shall find they 



may be, in the present state of our knowledge, so retained in 



classification. 



I. 



Limulus (Fossil and living). 



1. Eye.s sesijile, compoimd. 



2. Ocelli distinctly seen. 



3. All the limps serving as mouth- 

 organs. 



4. All the thoracic segments bear- 

 ing the branchiee or reproduc- 

 tive organs. 



5. Other segments destitute of any 

 appendages. 



6. Thoracic segments anchylosed. 



7. Abdominal segments anchylosed 

 and rudimentary. 



8. Metastoma, rudimentary . 



PterygotHs (Fossil, extinct). 



1. Eyes sessile, compound. 



2. Ocelli distinctly seen. 



3. All the limbs serving as mouth- 



organs 



4. Anterior WiOv?iC\c segments bear- 



ing branchice or reproductive 



organs. 



5. Other segments destitute of any 



appendages. 



6. Thoracic segments unanchylosed. 



7. Abdominal segments free and 



well developed. 



8. Metastoma, large. 



