THE BNTEKOPNEUSTA. 41 



wards; secondly, that as a matter of fact, in B. Robinii at 

 all events, the ectoderm cells may themselves be traced into 

 tails of this kind ; thirdly, the general absence of nuclei in 

 the "punktsubstanz," for if these fibres are supporting cells, 

 nuclei might be expected to be found in their course ; foxirthly, 

 there is an k priori difficulty as to the nerve supply to the 

 muscles in these animals, for, though the body of some of the 

 species is very thick, no definite nerve-cords are to be 

 found crossing the body cavities, with the exception of the 

 " dorsal roots " mentioned hereafter. How, then, are the 

 muscles innervated ? It seems, then, at least possible that 

 the nerve supply is derived directly from the skin, in which 

 case the fibres leaving the " punktsubstanz " naturally suggest 

 themselves as the transmitting agents. Finally, the view that 

 these fibres are ectodermic is rendered likely from the fact 

 that their origin may occasionally be traced from a very high 

 level in the skin, though the appearance which is sometimes 

 produced in sections as of their actual continuity with the 

 undoubted ectoderm cells may not be quite reliable. In a 

 few instances these fibres appear to anastomose with meso- 

 blastic elements, though this cannot be quite definitel}'^ 

 affirmed. On the whole, the balance of evidence seems in 

 favour of the view that they are ectodermic. If this be 

 correct the skin of Balanoglossus is to be regarded as a 

 collection of sensory cells ending in long fibres, which may 

 either be connected to the central nervous system, pro- 

 bably by the longitudinal fibres of the " punktsubstanz,'' 

 or may pass directly through this as motor fibres into the 

 muscles. 



The next point relates to the question as to the intervention of 

 some third cell in their course functioning as a ganglion cell. In 

 B. Robinii, in which the examination of this subject is most 

 complete, as stated above, the occurrence of such cells could 

 not be shown ; but this is, of course, by no means conclusive 

 in face of the antecedent probability of their occurrence. The 

 "punktsubstanz," then, would mainly consist of atFerent fibres 

 passing to the central nervous system, and the motor fibres 



