ON THE DIPLOOHORDA. 407 



Ectoderm. 



The ectoderm covering the trunk is corrugated (fig. 55) 

 into circular ridges, which have been supposed to be due to 

 contraction caused by spirit. An inspection of the figure 

 would lead one to doubt this, as the reduplications appear to 

 be very regular, and the thin cuticle runs from tip to tip of the 

 corrugations instead of following the course of the corruga- 

 tions. The cells are elongated and have elongated nuclei. 

 On the left hand the presence of black pigment spots is 

 indicated. At the base of the ectoderm cells is seen the fine 

 plexus of nerve-fibres which is very characteristic of Plio- 

 ronis as of Balanoglossus. Below this again is the 

 chondroid tissue. 



At the junction of collar and trunk the nerve-plexus be- 

 comes hypertrophied into a massive post-oral ring of nerve- 

 cells and nerve-fibres (fig. 56). The inner ends of the 

 ectoderm cells can be traced in some cases into nerve-cells, 

 and in others into nerve-fibres. The nerve-cells rather tend 

 to accumulate close under the chondroid tissue, but every 

 transition stage can be selected from the densely crowded 

 long ectoderm cells to the nerve-cell removed from the 

 surface. Fig. 57 shows some of the transition stages ; they 

 indicate how intimately the nervous system still is bound up 

 with the ectoderm in this group. 



The ectoderm of the collar or lophophore is of two types, 

 which I have elsewhere described as branchial and atrial 

 epithelium respectively. 



From other work upon the alimentary processes of Echi no- 

 derm and other larvae I have been led to more generalised 

 names for these types of ectodermal epithelium. A great 

 number of facts seem to point to the supposition that the 

 method of food ingestion, by the activity of ciliated areas 

 causing currents of water, which in their turn carry micro- 

 scopic food particles, is the primitive method for the early 

 Metazoa from the gastrula stage onwards. This method of 

 food ingestion may be termed cilio-trophic, and is found 



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