ix ENTEROPNEUSTA ALIMENTARY CANAL 565 



one or more of the most anterior are really hollow, containing an axial canal. 

 The posterior roots are solid strands of epithelium. No outer aperture of the axial 

 canal has been observed, although the root tissue passes directly into the body 

 epithelium, and the limiting membrane, elsewhere found below the epithelium, is 

 interrupted at the point where they join. On the peripheral, outer side of the 

 roots, the plexus of nerve fibres of the integument is continued into the nerve tissue 

 of the collar cord. The roots themselves are connected only with the dorsal layer 

 of cells of that cord. 



The collar cord contains cavities : these are either numei-ous small medullary 

 cavities, arranged more or less exactly in two longitudinal rows, or they form one 

 continuous central cavity, an axial canal (Ptychodcra) which either (in one species) 

 opens outward at the anterior and posterior boundaries of the collar region, or (in all 

 other species in which this point has been investigated) ends blindly at these points. 



The axial canals of the roots of the collar cord (which run in the dorsal mesentery) 

 are in communication either with the axial canal of that cord or with its medullary 

 cavities. 



IV. Sensory Organs. 



Even the most recent careful investigations have not been able to 

 demonstrate with certainty the existence of specific sensory organs. 

 Undifferentiated sensory cells may be scattered over the whole of the 

 integument. At the posterior, and especially at the postero-ventral 

 part of the proboscis, and, further, at the anterior edge of the collar, 

 the constitution of the body epithelium is such as to make it highly 

 probable that it is a sensory epithelium. In one species alone, 

 Balanoglossus canadensis, in the postero-ventral wall of the proboscis, 

 there is a deep epidermal pit, which is the only structure that can, 

 with any probability, be claimed as a localised sensory organ. 



On the sensory organs of the free-swimming larva, cf. the section 

 on Ontogeny, p. 586. 



V. The Alimentary Canal. 



The alimentary canal runs as a large and usually straight epi- 

 thelial tube through the body, from the wide oral aperture, at the 

 anterior and ventral end of the collar, to the terminal anus. It is, as a 

 rule, attached to the body wall by both a dorsal and a ventral mesentery 

 traversing the body cavity. It is developed in ways peculiar to the 

 different regions of the body. Especially noteworthy is the fact that, 

 in the branchial region, it forms a branchial intestine, communicating 

 by means of two longitudinal rows of branchial canals (gill-slits) with 

 the exterior. 



A. The mouth is followed by the spacious bueeal cavity, which 

 traverses the collar region, and is provided with a thick epithelial 



wall. 



B. The roof of the buccal cavity grows out to form a diverticulum 

 directed anteriorly ; this runs through the neck of the proboscis, 



