NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS yj 



secondarily developed in connection with metamerism; (6) internal 

 cleavage of the mesoderm segments to form paired coelomic cavities ; 

 (7) a somatic muscular system applied against the body wall, con- 

 sisting primarily of an outer set of constrictor fibers running in 

 transverse planes, and of an inner set of contractor fibers taking a 

 longitudinal course, each of which may be variously amplified or 

 reduced ; (8) the development of a blood vascular system from the 

 mesoderm, composed essentially of a dorsal and a ventral longitudinal 

 vessel connected by lateral vessels, but often reduced to a dorsal 

 vessel and more or less well-defined sinuses; (9) the association of 

 the germ cells with the walls of the coelomic sacs, and their discharge 

 into the coelom. 



The common basic features of organization above enumerated 

 attest the origin of the Arthropoda, the Onychophora, and the higher 

 Annelida from a common ancestral form, which itself must necessarily 

 be visualized as a generalized annelid. It is to be assumed that the 

 progenitors of the three groups had already acquired a lengthened 

 body by the addition of secondary genital somites proliferated from 

 a subterminal zone of growth. Though teloblastic growth does not 

 appear in the ontogeny of the Onychophora, it is quite as character- 

 istic of certain arthropods as of the annelids. 



The Arthropoda have in common with the Onychophora the follow- 

 ing nonannelid characters : ( i ) A chitinous ectodermal cuticula ; 

 (2) segmental ambulatory appendages formed as simple outgrowths 

 of the body wall, which in their structure and development give no 

 suggestion of a community of origin with the composite parapodia 

 of the Polychaeta; (3) segmental excretory organs (antennal, maxil- 

 lary, and coxal glands) that resemble the nephridia of Onychophora 

 in being remnants of coelomic sacs connected with the exterior by 

 simple coelomoducts, but which have neither the anatomical position 

 nor the development of annelid metanephridia ; and (4) closed gona- 

 dial sacs of coelomic origin, containing the germarial centers in their 

 walls, and connected with the exterior by a pair of coelomic sacs set 

 apart to serve as genital ducts. A feature characteristic of both the 

 Arthropoda and the Onychophora is the restoration of the haemocoele 

 as the definitive body cavity, resulting from the reduction of the 

 coelom to the cavities of gonadial and nephridial sacs, but it is not 

 distinctive of them because an obliteration of the coelom occurs also 

 in certain annelids. 



The small but important assemblage of characters given above as 

 common to the Onychophora and the Arthropoda would seem to in- 

 dicate that the two groups have been evolved from the same ancestral 



