ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 465 



been placed incorrectly by all students of leech metamerism, with the 

 possible exception of Vaillant (1870) in tbe case of a single genus 

 (Pontobdella), from the time of Gratiolet (1852) to the present. 



The natural and true limits of the somite coincide with the limits of 

 the neuromere ; that is, a somite includes those annuli which typically 

 are innervated from the same nerve-ganglion. This is confirmed by an 

 examination of metamerically repeated structures other than ganglia, 

 namely, septa, testes, and crop diverticula. Neuromeric groups of rings, 

 that is, somites as above defined, behave as structural units — (a) in somite 

 abbreviation (reduction in the number of rings in a somite), aud (b) in 

 somite elongation (increase in the number of rings in a somite). Both 

 reduction and increase in the number of rings take place chiefly at the 

 ends of the somite. The sensory ring occupies the middle of the somite, 

 and is least often and least extensively affected in the two processes just 

 named. It represents the stable component of the somite. 



The five-ringed type of somite found in the Gnathobdellidfe has been 

 derived from the three-ringed type found in the Khynchobdellidro, as 

 suggested by Whitman and demonstrated by Bristol. This has been 

 brought about by division of the non-sensory ring at either end of the 

 somite. 



The wide prevalence of ring multiplication among the Hirudinea sug- 

 gests the derivation of the three-ringed type of somite from a still simpler 

 type, consisting, as in Chretopoda, of a single ring. A phylogenetically 

 intermediate stage between the one-ringed and three-ringed types of 

 somite is probably represented in a typical body somite of Branchio- 

 bdella. The same type of somite structure appears also in the abbreviated 

 somites of Glossiphonia, as a stage intermediate between the three-ringed 

 and one-ringed conditions of the somite. The phenomena of ring multi- 

 plication in the Hirudinea is correlated with the restricted number of 

 somites found in the body. Increase in the number of somites does not 

 take place in the body of the adult leech. Without this, elongation of 

 the body is possible only through lengthening of individual somites. 

 Lengthening of the individual somite has probably been the cause, phylo- 

 genetically, of increase in the number of rings in a somite. 



Nematohelminthes. 



Phagocytosis in Nematodes.* — Prof. N. Nassonow describes, in 

 Ascaris osculata, A. ferox, Strongylus paradoxus, and Sclerostomum 

 armatum, unicellular phagocytic organs like those described in A. mega- 

 locephala and A. lumbricoides. They occur attached to the body wall, 

 aud often, though not always, bear an extraordinary array of amoeboid 

 processes. 



Platyhelminthes. 



Regeneration in Planarians.f — Mr. F. E. Lillie discusses, in par- 

 ticular, the source of material of new parts and the limits of size. He 

 studiel the well-known tendency of Planarians kept without food to 

 diminish in size. The smallest specimen obtained (after 43 days) was 

 less than one-hundredth the bulk of the original animal. When it had 

 been reduced to less than one-half of its original length, it was cut in 



• Arch. Mikr. Anat., lv. (1900) pp. 488-513 (3 pis.), 

 t Amer. Nat., xxxiv. (1900) pp. 173-7. 



