WITH NOTES ON THE WEST INDIAN SPECIES. 



309 



the neuro-hypophysial canal; the proboscis-pore of Enteropneusta is represented in the 

 Ascidian larva by the pore leading from the neuro-hypophysial canal into the cerebral 

 vesicle; the anterior neuropore of Enteropneusta is represented in the Ascidian larva by 

 the pore leading from the neuro-hypophysial canal into the mouth. 



y. Association of mouth and neuropore. We have already dealt with the association 

 of proboscis-pore and neuropore. It now remains to say a few words on the association 

 of mouth and neuropore in order to appreciate the comparison between the anterior 

 trematic complex (mouth, neuropore and proboscis-pore) of the Enteropneusta and of 

 the Ascidian tadpole. 



For my part, when I say that the mouth of the Ascidian tadpole is dorsal and the 

 mouth of the Enteropneusta is ventral, I mean that in one case the mouth is on one 

 side of the body, and in the other it is on the opposite side of the body. 



The comparison may be tabulated as follows: 



Enteropxeusta. 



1. Mouth ventral. 



2. Mouth a vast crescentic or sub-circular 

 orifice facing forwards. 



3. Mouth indirectly associated with neuropore 

 in virtue of the projecting collar-flap (PI. XXVII. 

 Fig. 6 a). 



4. Food ingested as the animal burrows through 

 sand, vast quantities of which pass through the ali- 

 mentary canal where the nutritious matter is ex- 

 tracted and the sand rejected. 



Ascidian Larva. 



1. Mouth dorsal. 



2. Mouth a minute circular orifice facing up- 

 wards. 



3. Mouth directly associated with neuropore, 

 the latter opening into the buccal cavity. 



4. Doubtful if larva feeds at all ; in many cases 

 it certainly does not. If it does the food (as in 

 adult) is ingested by ciliary currents and consists of 



-organisms and organic debris. 



In the locomotion of the Enteropneusta the muscular proboscis is the essential organ 

 of burrowing, and the distensible collar the essential organ of progression 1 . 



The passage of the mouth from the ventral position which it occupies in the 

 Enteropneusta to its dorsal position in the Ascidian larva may perhaps be attributed 

 physiologically to the increased importance and efficiency of that association of mouth 

 and neuropore which may be said to be already foreshadowed in the Enteropneusta (see 

 PL XXVII. Fig. 6 a). The practical difficulty in the way of this translation of the 

 mouth presented by the proboscis intervening between the latter and the dorsal surface 

 can be supposed to have been, and probably was, surmounted pari passu with the change 

 of function of the proboscis from a muscular burrowing organ to a non-muscular 

 snout or fixing organ. This change would carry with it changes in the entire order of 

 development and the mouth could open dorsally coincidently with the neuropore before 

 the formation of the praeoral lobe. This is essentially what does happen 2 . 



1 A remarkably pretty analogous method of locomotion is exhibited by Dentalium, a species of which I had 

 the opportunity of observing at Lifu. Here the muscular foot with its pointed end is the essential organ of 

 burrowing, while the two lateral aliform lobes, which expand at the proper moment, together constitute the 

 essential organ of progression. 



s See A. Willey, "Studies on the Protochordata," I. Q. J. M. S„ Vol. xxxiv., p. 317 and IH. Ibid. Vol. 

 xxxv., p. 316. My interpretation of the organ of fixation of the Ascidian larva as praeoral lobe has been met 

 with some natural scepticism, but on this point I may say that my views remain unchanged. As it happens, 

 however, this is a point of detail, and we can go a long way without it. 



