540 E. RAY LANKESTER. 



of some Arthropods, sucli as tlie hermit crabs and the spiders. 

 Admitting that the Echiurids are descended from Chtetopoda, 

 such a change has taken place in them amounting to little 

 short of complete lipomerism, though not absolutely complete. 



Eecent suggestions as to the origin of the Mollusca involve 

 the supposition that such an efPacement of once well-marked 

 metamerism has occurred in them, leaving its traces only in 

 a few structures such as the multiple gill-plumes and shell- 

 shields of the Chitons and the duplicated renal sacs of 

 Nautilus. 



A further matter of importance in this connection is that 

 when the old metameres have been effaced a new secondary 

 segmentation may arise, as in the jointed worm-like body of 

 the degenerate Acarus, Demodex f olliculorum. 



Such secondary annnlation of the soft body calls to mind 

 the secondary annnlation of the metameres of leeches and 

 some earthworms. Space does not permit of more than an 

 allusion to this subject, but it is worth while noting that 

 the secondary annuli marking the somites of leeches and 

 LumbricidaD in definite number and character are perhaps 

 comparable to the redundant pairs of appendages on the 

 hinder somites or Apus, and are in both cases examples of 

 independent repetition of tegumentary meromes — a sort of 

 ineffectual attempt to subdivide the somite which only pre- 

 vails on the more readily susceptible meromes of the integu- 

 ment. 



The development of secondary metameric annulatious 

 within the area of a complete somite is not recorded among 

 Arthropoda. It deserves distinct recognition as " hypo- 

 metamerism" or formation of '^ somatidia." 



The last law of metamerism which we shall attempt to 

 formulate here, as the Thirteknth, relates to the fusion or 

 blending of neighbouring somites. There are, without doubt, 

 a large number of important generalisations to be arrived at 

 hereafter from the further study of the metamerism of 

 Vertebrata and the peculiar phenomena exhibited by tlie 

 dislocated meromes of the vertebrate's somites. But this is 



