316 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



Baldwin Spencer compared the eye of the Ascidian tadpole with the pineal eye, 

 the most tangible point of resemblance being in the method of formation of the lens, 

 the entire eye, lens and retina being of myelonic origin. This comparison may still 

 hold good so far as it goes. But the full significance of the pineal eye can only be 

 appreciated when we have traced its origin, or at least its affinity, to something which 

 was not an eye at all but a far more generalised sensory apparatus. 



Moreover the result of recent work 1 (Klinckowstrom, C. Hill, etc.) has been to 

 show that the epiphysial complex is not a simple outgrowth from the roof of the 

 primary fore-brain ; there may be more than one outgrowth ; there may even be more 

 than one pineal eye with retina and lens complete. 



Without going into greater detail, I may refer the reader back to my account of 

 the roots in Pt. Jiava (p. 234), the intra-epidermal canals in Pt. carnosa (p. 252), and 

 the vestigial root of Sp. porosa (p. 271). 



I have described the remarkable terminal bulb of the vestigial root of Spengelia 

 porosa (PI. XXXI. Fig. 40) as being in a condition of mucoid degeneration. Whether 

 or not there is any trace of pigment in life I cannot say. Very probably there is 

 some kind of colouring matter. But we do not expect to find any great display of 

 pigment in the Enteropneusta since they are burrowing creatures, living concealed from 

 the light of the sun and belonging essentially to the marine cryptozoic fauna. If Sp. 

 porosa lived an exposed life the terminal bulb would probably be in a condition of 

 pigmentose degeneration. 



An epiphysial structure like an enteropneustic root can be transformed into an 

 epiphysial structure like a pineal eye by losing its primary function, passing through 

 a condition of pigmentose degeneration (or at least developing pigment in its walls) 

 and then being rejuvenated by the acquisition of a new function, the agent of the 

 rejuvenescence being some form of natural selection. 



We may therefore infer the following cycle of events : — 



I. II. III. IV. 



Enteropneustic roots Pigmentose Pineal Epiphysis 



(Ptychoderidae) condition eye or eyes cerebri 



I will now state with confidence the following proposition. 



The medullary tube of the collar of Enteropneusta is the homologue of the cerebral 

 vesicle only of Amphioxus and of the Ascidian tadpole and probably represents no more 

 than the primary fore-brain- (thalatnenceplutlon) of Craniota ; the roots 3 of Ptychoderidae 



1 A. Klinckowstrom, " Beitrage zur Kenntniss des Parietalauges," Zool. Jahrb. (Abth. f. Anat. u. Ont.) vn. 

 1894, p. 249; Charles Hill, "The epiphysis of Teleosts and Amia," Journ. Morph. ix. 1894, p. 237; W. A. 

 Locy, "Contribution to the structure and development of the Vertebrate Head," Journ. Morph. xi. 1895; see 

 remarks on the Pineal Sense-organs, p. 561 and bibliography; also A. Prenant, Elements d'embryologie...des 

 vertebres, Vol. n. p. 566 et seq., Paris, 1896. 



2 Whether or not it contains elements of the mesencephalon need not be discussed here. The funda- 

 mental truth is that the primitive cerebral vesicle has been closed in phylogenetieally in advance of the 

 spinal cord, which meanwhile is represented by the dorsal nerve tract in the skin. 



3 I have not considered the origin of these roots themselves. All the facts which are known (e.g. the 

 appearance of a median neural crest in Pt. fiava (above, p. 235); the median keel observed by Spengel in 



