542 E. KAY LANKESTER. 



shields of many Trilobites. On the other hand, the telson, 

 which is joined in both these cases with the superficially 

 fused segments by a fusion of its chitinous cuticle with that 

 of its last-formed or budded somite, can only take part in 

 the fusion as a result of arrest in its activity, which amounts 

 to a late supervening atrophy. This arrest of the telson's 

 special bud growth may take place very early, in which case 

 we get a large telsonic shield and only a very few somites in 

 front of it — none soldered to the telson as in Agnostus and 

 Ilenus ; or it may take place later when eight post-cephalic 

 (opisthosomatic) somites have been formed as in Limulus — 

 the last two incompletely. Or, again, thirty or more somites 

 may have been produced before the arrest takes place, and 

 fifteen of these may be ankylosed with the telson to form the 

 pygidial shield (Phacops, etc.). 



A more complete fusion of somites is that seen in the head 

 of Arthropoda. The head or prosoma of Arthropod a is a 

 tagma consisting of one, two, or three prosthomeres or 

 somites in front of the mouth, and of one, two, three, up to 

 five or six opisthomeres. The cephalic tagma or prosoma 

 may thus be more or less sharply divided into two sub- 

 tagmata, the prfe-oral and the post-oral. 



The shifting of the mouth backwards in Arthropoda so as 

 to allow segments which once were post-oral to take up a 

 prseoral position, as prosthomeres, must be regarded as a 

 case of dislocation of the meromes concerned (sixth law), 

 like the forward travelling of a fish's pelvic fins. The anus 

 does not appear to be liable to such dislocation in Arthro- 

 poda, but it certainly does travel away from its parental 

 metamere in the Verbebrata, and may possibly do so in 

 Chaitopoda when what must be called ''lipomerism " or 

 general obliteration of a metameric ordering of parts sets in. 

 Such " lipomerism " must be supposed to have aifected the 

 Cha3topod ancestors of the Sipunculids, if those latter worms 

 are to be traced genetically to the former, and the anus has 

 shifted to the anterior third of the body. However that may 

 be, the conception (first put forward by Lankester in 1875) 



