No. ()2.j] . IHE KVOU TlnX OF A RT IllWPODS 147 



even further down in tlie developmental scale, than the 

 points of origin of the lines of descent of the Hemichor- 

 data and the worm-like forms. It is also possible to 

 suppose that in the hemicliordatan line there have ])een 

 carried over certain developmental tendencies from their 

 common ancestry, such as the preservation of a "tor- 

 naria" larva characteristic of the Ecliinodonnatn. while 

 in other respects the Hemichordatan line of (l(»vcloi»)u*nt 

 has paralleled that of the worm-like foi'iiis iikh c do-oly. 

 having taken over more of the tendencies wliieli wen- to 

 find opportunities for fuller expression in the wonii-iike 

 forms from their ultimately common inheritance. Under 

 these conditions the Hemichordata are related to both 

 the Echinodermata and the worm-like forms, but their 

 line of development has accompanied that of the worm- 

 like forms much more closely and for a longer distance 

 before each branched off along its own path of speciali- 

 zation. If the lines of development of the Hemichordata 

 and of the worm-like forms have an ultimately common 

 ancestry, and if both lines of descent have "travelled 

 along the same developmental road" for a considerable 

 distance before eaeli bi-an.-lied off ah)iig its own path of 

 specialization, it is not sni piMsin.o- that we iind many 

 structural resenil)hui(H'< in tlie two lines of de-cent, and 

 the rosembh-ince of -u.-h I lenii.-h.)rdata a- C pJinhulism^, 

 Bhalu/npJrnni, l>hnrn>ns, vU:. to .Tilain -\\^yv^K^r may 

 be as mneli the resnU of .■on>an,uninity a- of - .-on vn-ent 

 development." Tliis view enahU- n- to harmoni/e the 

 apparently discordant theories concerninu' the ultimate 

 origin of the Vertebrata — all of which may contain a por- 

 tion of the truth, as is frequently the ca>e in the different 

 hypotheses put forward to explain (MM-tain observed jilie- 

 nomena. Thus, accord inu- to this <M.nce])tion. we inay 

 derive the Vertebrata from fornw similar to the Hemi- 

 chordata, and still acctmnt for the anneli<l-like (and 

 arachnid-like) feature^ wliich ap]>eai- in cei'tain of the 



tendencies ])resent in the ancestors which ultimately 



