56 



REPORT 1871. 



To take a familiar iustance : if w^'compare the larval stages of the Com- 

 mon Shore-Crab {Carcinus 7noenas) with. Ptert/r/otus, 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 appendages, which are attached 

 to the mouth ; the abdominal segments 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 naturahsts 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 of Limulus and Pterygotus side by side, and 

 also those of TrUobita and Isopoda, we shall find they may be, in the present 

 state of our knowledge, so retained in classification. 



1. 

 2. 

 3. 



4. 



5. 



6. 



7. 



8, 



Pterygotus (Fossil, extinct). 



Eyes sessile, compound. 



Ocelli distinctly seen. 



All the limbs serving as mouth- 

 organs. 



Anterior thoracic segments bear- 

 ing branchiae or reproductive 

 organs. 



Other segments destitute of any 

 appendages. 



Thoracic segments unanchylosed. 



Abdominal segments /reeancZ well 

 developed. 



Metastoma large. 



1. 



2. 

 3. 



4. 



6. 



Limidus (Fossil, and living). 



1. Eyes sessile, compound. 



2. Two ocelli distinctly seen. 



3. All the limbs serving as mouth- 



organs. 



4. All the thoracic segments bear- 



ing branchiae or reproductive 

 organs. 



5. Other segments destitute of any 



appendages. 



6. Thoracic segments anchylosed. 



7. Abdominal segments anchylosed 



and rudimentary. 



8. Metastoma rudimentary. 



II. 



Trilohita (Fossil, extinct). 



Eyes sessile, compound. 



No ocelli visible. 



Appendages partly oral, partly 

 ambulatory, arranged in pairs. 



Thoracic segments variable in 

 number, from 8 even to 28, free 

 and movable (animal sometimes 

 rolling into a ball). 



Abdominal series coalesced to 

 form a broad caudal shield, 

 bearing the branchiae beneath. 



Lip-plate tuell developed. 



Isopoda (Fossil, and living). 



Eyes sessile, compound. 



No ocelli visible. 



Appendages partly oral, partly 

 ambulatory, arranged in pairs. 



Thoracic segments usually seven, 

 free and movable (animal 

 sometimes rolling into a ball). 



5. 



Abdominal somites coalesced, and 

 forming a broad caudal shield, 

 bearing the branchiae beneath. 

 G. Lip-plate small. 



Should our further researches confirm Mr. Billings's discovery fully, we may 

 propose for the second pair of these groups a common designation, meantime 

 we give the above as representing the present state of our knowledge. 



