642 PROFESSOR E. RAY LANKESTER, 
mouth and in some degree in crushing such food (as seen 
in Apus), but this development was specially carried out 
and localised in two pairs of appendages posterior to the 
one already so differentiated in Peripatus. 
The segments, each with its pair of appendages, were 
indefinite in number and frequently exceeded one hundred. 
The definite Crustacean character was attained when two 
pairs of appendages had become pre-oral, at least three 
pairs specialised as jaws and no longer locomotor, whilst 
the remaining appendages retained locomotor, manducatory, 
and respiratory functions to be subsequently specialised in 
the further development of the Crustacean stem. 
It appears to me probable that the Merostomata, including 
under this head the Xiphosura (Limulus), the Trilobita, and 
the EKurypterina, diverged from the main stem! of the Arthro- 
pod pedigree at a point between that indicated by the grade 
of organisation of Peripatus and that occupied by the Pro- 
Phyllopoda or earliest Crustaceans. 
Probably none of the known Merostomata suffice to give 
us a true picture of the structure of the ancestral Merosto- 
mata from which they are all derived. Probably these 
ancestral Merostomata were devoid of the prostomial an- 
tenne—the non-appendicular antenne. At the same time 
none of their post-oral appendages had become definitely 
pree-oral in position and nerve supply, though not less and 
probably not more than six pairs of pediform appendages 
were closely set round the mouth, their bases acting as 
powerful manducatory organs. To this group of appen- 
dages, of which the corresponding segments were more or 
less completely fused with the prostomium (forming the 
prosoma), succeeded a mid-region of the body (the mesosoma), 
consisting of numerous segments carrying biramose, pro- 
bably pediform appendages, the bases of which were beset 
with respiratory lamelle. 
The generative apertures were situate in the first or one 
of the anterior segments of this mid-region of the body. 
A third region of the body (the metasoma), also consisting of 
numerous segments, was probably distinguished by the form 
and smaller size of its appendages and by a tendency of the 
segments to fusion. Posteriorly to the anus was a median 
plate or spine. Probably the eyes placed on the dorsal sur- 
1 T have treated the line of descent leading to the Crustacea as the 
main stem of the Arthropod family-tree; it is obviously a matter which 
may be determined by convenience as to whether one or other of the 
a ea of a genealogical tree shall be treated as the main line of the 
family. 
