THE STRUCTURE AND SOME ASPECTS 



OF DEVELOPMENT OF THE 



ONYCHOPHORAN HEAD 



By F. H. butt 



Cornell University 



Ithaca, N. Y. 



The position of Peripatus relative to the arthropods on the one hand 

 and to the annehds on the other has led to an amount of attention 

 paid to this animal all out of proportion to its inoffensive, retiring, 

 and unspectacular habits. The reasons for this attention are not 

 difficult to understand when one considers that it apparently repre- 

 sents a link between two very important phyla. 



The characters wherein Peripatus appears to be close to the arthro- 

 pods are striking and significant. The body wall is similar to that of 

 many arthropods. The appendages diverge from the chaetal type 

 found in the annelids and exhibit more the embryological and later 

 developmental growth found in the arthropods. In fact, Snodgrass 

 (1938) considers the walking leg of Peripatus to be the prototype of 

 the arthropod limb. 



Peripatus has on the other hand definite relationships with the 

 annelids. It is wormlike with no distinct body regions. In the adult 

 stage it has no definitely segmented areas, though it has a head region 

 distinct in function from the rest of the body. Internally the nerve 

 cord is distinctly similar to the annehd type, though the anterior end, 

 especially the brain, shows some advances in structure. 



Internally, the alimentary canal, especially in the anterior end and 

 the middle sections, and the peritrophic membrane, especially in its 

 origin, are strikingly similar to those of the insects. The heart also 

 exliibits the more simplified form found in insects and in some other 

 arthropods. Is it any wonder, then, that over the long period of years 

 during which workers have investigated this animal many of them 

 have considered it to belong to the phylum Arthropoda? 



I do not wish, within the limits of this paper, to make an extended 

 study of the literature on Peripatus. That has been thoroughly done 

 by several others, and those who wish to pursue that subject further 

 I refer to the bibliographies in the works of Snodgrass, Man ton, and 

 Weber. 



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