Evolution of Animals 421 



anterior pore, the condition would be as seen in the Meta- 

 nemertinea, but were the pharangeal orifice to be carried for- 

 ward outside the oral area the typical proboscis sheath or 

 rhyncocoel and associated proboscis of other Nemertinea 

 would result. Various stages in this process can be traced 

 that are all illustrated and described by Burger. 



Phylogenetically then, we would regard the simple pro- 

 trusible pharynx of some Rotifera as the primitive rudiment 

 of the proboscis. In the simpler fresh-water Turbellaria or 

 Rhabdocoela, that show in many details characters intermedi- 

 ate between Rotifera and Nemertinea, the deep dorsal phar- 

 yngeal invagination with muscular wall and evertible proboscis 

 would represent the next stage (Plate Fig. 15a). By back- 

 ward prolongation of the invagination or coecum, and forward 

 traveling of the orifice till it lay outside of and in dorsal front 

 position to the mouth, the typical sheath and proboscis of 

 some nemerteans would result. This would be stimulated 

 to development on the principle of action and reaction — 

 stimulation flows of energy and stimulatoiy response — through 

 increasing sensitivity and elongation of the proboscis, and 

 its increased use as a tactile, an offensive, and an absoiptive 

 organ. In the Turbellaria all of these functions, specially 

 the tactile and absorptive, are clearly shown. But in the 

 Nemertinea the proboscis became more and more an elongate 

 sensitive rapidly expanding and contracting structure, that 

 has brought the organism into constant contact with environal 

 conditions. So it acquired a veiy fundamental association 

 mth, and became richly innervated by processes of, the brain 

 between which it lies in all higher nemerteans. Through 

 constant muscular activity also, in relation to nerve supply, 

 a steady increase in its investing sheath took place, till this 

 became the long hollow tube or rhyncocoel that stretches 

 backward and is a powerful support for the body parts. 



But its possible future evolutionary history gives to the 

 combined proboscis and proboscis sheath a surpassing interest. 

 In the years 1883 and 1885 the distinguished Dutch zoologist 

 Hubrecht promulgated the view that the latter is the pre- 

 cursor and first foundation of the vertebrate notochord. 



His hypothesis was thus expressed: "According to my opin- 

 ion the proboscis of the nelnerteans, which aiises as an invag- 

 inable structure (entirely derived, both pliylo- and ontogenet- 

 ically, from the epiblast), and which passes through a part of 

 the cerebral ganglion, is homologous with the rudimentary 

 organ which is found in the whole series of vertebrates without 

 exception — the hypophysis cerebri. The proboscidean sheath 



