260 WM. E. RITTER. 



anterior rim of the collar. By the contraction of these muscles 

 when the proboscis acts as a fixed point through the pressure of 

 the blebs above described against the sides of the burrow, the 

 collar and, of course, the whole pharyngeal-abdominal portion 

 of the body, are drawn strongly forward. Following, or coinci- 

 dently with, this movement, contraction of the longitudinal mus- 

 cles of the last-mentioned parts themselves takes place which 

 contributes still further to the advancement of the creature as a 

 whole. Circular somatic muscles are practically wholly wanting 

 in the collar. A few fibers have been described in some species, 

 but I am unable to recognize any excepting a few in the anterior 

 rim, in any of the species examined by me. 



From the account here given it is seen that we have in these 

 animals a system of locomotor muscles acting on an axial skeleton, 

 derived in large part from t/ic digestive tract. That the combined 

 notochord, consisting of the proboscis and esophageal portions, 

 and nuchal skeleton, must be interpreted as a unit of structure 

 with the office of an axial skeleton, I hope to be able to show con- 

 clusively in my forthcoming monograph of the Enteropneusta 

 of the Pacific Coast. I imagine, however, that the proposition 

 will be readily granted by most zoologists, particularly if the 

 facts here briefly set forth be considered along with others that 

 may be gathered from previous publications on the anatomy and 

 development of these animals. 



If now the considerations relative to the skeleton and muscles be 

 regarded from the comparative standpoint, the most striking thing 

 that comes to view is the trenchancy with which they separate 

 the enteropneusta from all the >invertebrata. Nowhere in all the 

 variety of organization represented by this subkingdom do we 

 find locomotion accomplished by muscles attached either to the 

 intestinal tract or to derivatives of it. But the trenchancy with 

 which the structural relations here considered separate the enter- 

 opneusta from the invertebrata is no greater than that by which 

 they connect them with the chordata. 



An axial skeleton derived primarily from the intestinal tract 

 and serving as the attachment of muscles of locomotion is, as is 

 fully recognized, one of the very essences of chordate organiza- 

 tion ; and in spite of the difficulties in the way of establishing a 



