MR 



SOME LOWER ANNELID- ft J'. 



is smaller in Chmtogaster than in the higher Oligoch et t s and hence iV\ r socids lie 

 held together to constitute an individual. Were it still less, we mi In haw an A undid 

 of but one segment, resembling in this a Rhabdocele Torbcllarian, a * <jitta t or other 

 simple worm, having a head, digestive tract, prostomial and ventral ncn -<-an«'lia and 

 a pair of excretory segment-organs, with cephalic and abdominal bristles. Tha growth 

 of the head always in an anterior direction, as an addition to the otherwise completed 

 zooids of Chcetogaster, seems to show that the head is something distinct fr in th oiler 



n 



segments, and not comparable to them, whilst it also tends to explain each ordin.in 

 segment as an incomplete zooid, and is further connected as cause and efh t with tie- 

 subsequent separation of the zooid at the plane in which the tendency to anterior deve- 

 lopment was first sufficient to overcome the longitudinal cohesion. The anomal us 

 cephalic bristle-bundles of Chatogastermny be compared to, and are perhaps homologous 

 with, the temporary cephalic bristles of larval Polychseta, or with the jaw-like sct:o of 

 Sagitta; or, on the other hand, they may be related to the » v discovered b\ IVofessot 

 Huxley in connexion with the peristomial cirri of Polynoina. 



The Fissiparous Reproduction of Chsetogaster limns ei.— A few a >rds are necessan 

 as to the order of the production of zooids, and the relations of the "segment! " which 



aggregate, and not as the result of tertiary aggregation. The same reasoning wiD, y . v m applied to Um 



orming 



a 



gmentcd stem I m-il also sup- nudijoed, and tha 

 parts of the uni*egm<'nta] MKjondarj 



- ■ 



dim 



typical number of "somites" are much lessened. In the development of the zooid, I Oh « . the il» 

 bristles, pharynx, and surrounding parts are produced by growth in an anterior air on. tb rest F 1 ■ fart I 

 ment is completed by posterior growth, the two axes being separ, d by an indifferent plan, The fcnt aegm ,t 

 forms a simple secondary aggregate. This is exactly comparable to the , rly d. pment of ft. ar» of mann, 

 .__.«^ .A -* i^^. . i« t.h« former the rudiments of the pr< tomal and peristomal region, with At temp**! 



nae to th<' w«>rra 



sres 



rved to take place d h sooiw j 



(without the production of segments 



»-• * 



►mil which • 



rattan 1 



Dohrn 



to form the complex rajn.-ipou, ^iww «, 



The bud-somites in VhcttogtOttr, th. n. are produced posteriorly but 1 



incomplete, they do not develope a head : 

 Where first the longitudinal cohesion becon 

 head grows, and a new individual separates. 



iwl C"hesto 



anterior de lopment r< rails, and tie i . .rraal 

 s longitudinal cohesion counteracts rely tl 



Arthropod, th rr 



, j \.™A n^l to somrate. It is conceivawc mat » tu« «..»«« «v -• - r - 



opposing ten eney to produce a W -^"^ „, the ^ fora . „, b S i. cpo«d 



of the Annelid, or the pygidium is j 



is inch a rtruftup 



segm 



orm in to appendage, (in accordance with a repe.i.i.n f « nal n,rc, being 



traceable the reDetition of general lorm in ira app«.m"6~ v - 



traceaoie, tne reptuuuu e . g generally simp! 



the principal fact tending to indicate it, TrhiM the . ^ ^ ^.^ ^ 



a ifeZw, or the rows of branchiae in an Eohs, mi e ni 



the appendages alone of the head in Annulosa did so ^ ^ ^^ be , onsidored establish 



But if the identity of the anterior cephalic somites m ^ ^ ^^ ^ ^ rf , ^ ( jn ^^ 



by the common plan of their structure, we ™«^ ltmUqeM as fc «, a .?*** - * -* 



recorded by Le Vaillant and others, and must ***> ^ ^ ^ ^ . .^^^ , o ^ the gimpl . ^^ 

 gemmation, and not exchangeable with those ot ie J ja] an( , r t0 its individ.jal v (aa • 



. , . ., a ._j \ — ^1^1 «8 noaaessinff a «€«'' <>/ n * w < ' _ _ „ „ , __, „ , ^„\. rriin 



mental Arthropod and 



mental Arthropod aim aiu»w«. - r ; . . i Tom - Atl n f a 



secondary aggregate) as the head, or pro- and penpal rcg,on 



Mollusk 



4 



VOL. XXVI. 



