94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



possibly represented by the median tentacle of certain Polychaeta 

 (fig. 13 C), which, having a double nerve root in the brain (fig. 45 B, 

 C, iTlNv), might be supposed to have had itself a double origin. 

 However, the possibility of the median polychaete tentacle having 

 been formed by the union of a pair of apical tentacles is denied by 

 Binard and Jeener (1928). 



The theory here proposed to explain the occurrence of coelomic 

 sacs in the prostomial region of the articulate animals has no relation 

 whatever to the theory of Sedgwick (1884), Lameere (1926), and 

 Binard and Jeener (1928) that the annelids and arthropods are 

 derived from a coelenterate polyp form, and therefore have funda- 

 mentally a radial organization (fig, 40 A). A radial structure secon- 

 darily aflfects the anterior end of the articulate trunk because of the 

 subapical position of the mouth (B) ; but the terminal position of the 

 anus creates a quite dififerent structure at the posterior end. 



The term acron (Janet, 1899) is frequently used by students of 

 arthropod embryology to designate the apical part of the arthropod 

 head that lies anterior to the first true somite ; its exact application, 

 therefore, differs according to each writer's interpretation of the 

 head segmentation. Janet defined the acron as the preantennal part 

 of the head. As the term is used in the present paper, the arthropod 

 acron is equivalent to the annelid prostomium, and is represented in 

 the arthropod embryo by the cephalic lobe (or lobes) bearing the 

 eyes, the labrum, the preantennae, and the first antennae. The pro- 

 stomium is primarily the anterior part of the trunk not invaded by 

 the blastopore (fig. 6 D, Prst) ; the median part of the arthropod 

 acron is always preoral, but its lateral parts may lap backward and 

 extend even a considerable distance behind the mouth. The telson 

 at the posterior end of the trunk is not morphologically equivalent to 

 the acron. It is traversed by the alimentary canal, and has the anus 

 at its extremity ; it does not contain coelomic sacs, but its represen- 

 tative in the annelids, the so-called pygidium, may support a pair of 

 tentaclelike appendages. 



The principal reasons for regarding the oculo-antennal region of 

 the arthropod head, here defined as the acron, as representing a 

 primarily unsegmented archicephalon corresponding with the annelid 

 prostomium may be summarized as follows : ( i ) There is never any 

 external division of the acronal region into segmental areas; (2) there 

 is no specific evidence of the cephalization of primarily postoral 

 somites, except in the case of the tritocerebral somite; (3) the embry- 

 onic coelomic sacs of the first antennae, the preantennae, and the 

 labrum are formed directly where they occur in the cephalic meso- 



