ORGANS OF THE SPAT 63 



microscopic preparation, render it very difficult to obtain satisfactory 

 views in the living state. I have selected preserved specimens, made 

 sketches of them and taken measurements, and then prepared series of 

 sections of them, of full grown larvae that must have been on the point of 

 becoming fixed, of recently attached spat that were smaller than, the same 

 size as, or scarcely larger than the largest larvae, and succeeding stages up 

 to sizes big enough for dissection. I have sections of seven larvae vary- 

 ing about a length of 55, and spat of lengths 53 (= -365 mm.), 55 (= "379 

 mm.), 55, 55, 60, 62, 65, 67, 70, 72, (=-496 mm.), 80, 105, 110 (=-759 

 mm.), 110, 115, 145, (=1 mm.) and the following lengths in millimetres: 

 1, 1, 1-5, 2, 2, 2-5, 3, 3, 3-5, 4, 5, 5-5, 10-5, 21, 32, 47. One would think 

 that with this preparation the subject could be easily settled, as many a 

 subject might be, but those used to studying series of sections know that 

 with such minute objects there are great difficulties, one of which is the 

 obtaining of the sections symmetrically or in the required direction. Be- 

 sides, other complications arise that are not observable in living specimens. 

 Such a question as " Does the velum become changed into the palps?" be- 

 comes rather " Does any part of the velum furnish a starting point for the 

 production of palps?" The palps are oral organs and the mouth is but 

 the open end of the oesophagus, which latter is bound up with the velum, 

 being moved with it and associated with it in some of its activities. The 

 tissues between the oesophagus and velum are common to the two, and it 

 becomes difficult to decide to which they belong. 



A spat of length 53 (= -365 mm.) has still the structure of the larva, 

 and as such possesses a distinct velum (Plate VI, Figs. 11, 12), which was 

 not rejected bodily at the time of fixation. It is, however, somewhat 

 shrunken, collapsed, and of a vague structure, such as would suggest loss 

 of function. At a size of 55 the velum is so far gone as to be unrecognizable, 

 and at 62 there remain mere vestiges of it, chiefly of its muscles. It is 

 surprising how rapidly it disappears, for this small growth of spat can 

 represent but a few hours — a day at the most. The chances are that the 

 change is generally more rapid, for the spat of 55 length must be an 

 unusually young one, so that its velum may still have had a short lease of 

 life when fixation took place. 



While dealing with the fate of the velum and its relation to the palps 

 it might seem best to consider also the origin of the palps, since it is not only 

 necessary to prove that they are not transformed velum, but also, as an 

 additional proof, to show what they do spring from. But as they more 

 properly belong to the mouth their origin will be taken up with the in- 

 testinal system. 



The Foot persists into young spat stages, in which it may be followed, 

 in sections, to spat beyond 1 mm. in length. In the sections of my young- 

 est spat (which was scarcely over i mm. in length) it is present in un- 

 diminished size (Plate VI, figs. 12, 13, 14), though not so clear and definite 



