NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 9I 



(D, E, 2 Ant) take on the biramous structure characteristic of the 

 following somatic appendages. The postoral segment of the second 

 antennae is thus, according to Sollaud's interpretation, the first true 

 somite. The same view is strongly advocated by Holmgren (1916) 

 and Hanstrom (1928) from a comparative study of the annelid and 

 arthropod brain, but, as will be shown later, the evidence adduced by 

 these authors from the brain structure must be qualified by facts 

 of development. 



The principal ground for the generally accepted belief that the 

 acronal region of the arthropod embryo contains one or more "cepha- 



FiG. 39. — ^Diagrams of cephalization in the Polychaeta and Arthropoda, show- 

 ing the relation of the annelid prostomium to the arthropod head on the assump- 

 tion that the first antennae are prostomial appendages. 



A, an adult polychaete with prostomial tentacles and palpi, first two somites 

 united in the peristomium. B, an insect embrj'o in which the head (acron) is 

 an archicephalon representing the annelid prostomium, and may bear two pairs 

 of appendages. C, a theoretical protomandibulate arthropod, in which the head 

 is a protocephalon (Prtc) composed of the acron and one somite. D, a 

 chelicerate arthropod, in which the acron is extended laterally and dorsally 

 over several somites united in the prosoma. 



Acr, acron (arthropod prostomium) ; lAnt, first antenna (acronal appen- 

 dage) ; 2 Ant, second antenna (appendage of first somite) ; Chi, chelicera (equiva- 

 lent to second antenna) ; E, lateral eye; I-VI, first six somites; Lm, labrum ; 

 Md, mandible ; Mth, mouth ; iMx, 2Mx, first and second maxillae ; Pdp, pedi- 

 palp ; Perst, peristomium ; Pip, palpus ; Prnt, preantenna ; Prst, prostomium ; 

 Prtc, protocephalon; Tl, tentacle. 



lized somites" is the occurrence of temporary coelomic sacs in this 

 region. However, it has not been shown that the presence of cavities 

 in the cephalic mesoderm is necessarily indicative of somites, and it 

 would seem that the burden of proof should be on the positive side 

 of this question. 



The mesoderm bands of the annelids, as shown in an earlier part 

 of this paper, extend forward in the sides of the body from their 

 posterior centers of propagation. In the trochophore larva the meso- 

 derm is arrested at the mouth, but in the later development of the 

 worm the bands extend into the prostomium and may here contain a 

 pair of coelomic cavities. While it is usually observed that the pro- 



