34 WILLIAM BATESON. 
same age without it ; hence it may be inferred that it undergoes 
a rapid atrophy at this point. 
Similar suckers! occur as larval organs in Tunicata, Ganoids, 
and Amphibia. 
With regard to the meaning of the sucker, considering the 
time of life at which it appears, it is probably not ancestral in 
origin; as the animal is already, from Stage G onwards, a 
distinct Balanoglossus in all its characters. It is more rea- 
sonable to conjecture that it is of purely developmental 
importance, and indeed its use to a larva of this kind is suffi- 
ciently obvious; for the creatures inhabit shallow pools on the 
sand-flats, being just buried in the mud, from which position 
they would be in danger of being washed away by the incom- 
ing tide and so be dried up by the great heat of the sun at low 
tide. On attaining a larger size the body can be, and always 
is, coiled round a spindle of sand and thus is kept in position, 
hence the sucker is no more required. In connection with 
this sucker I observed that nearly all the animals found in 
these pools were such as are provided with similar means of 
fixing themselves, which power is probably essential to life in 
such a habitat. An account of the histology of this sucker will 
be given subsequently. 
During the period which elapses between the appearance of 
the first and second pair of gill-slits the body gradually 
acquires, as was mentioned above, a considerable degree of 
transparency. Owing to this fact several points of internal 
structure may be observed. ‘This is especially marked in the 
case of the alimentary canal, which can now be clearly per- 
ceived to consist of three regions—an anterior branchial tract, 
a middle digestive portion, and an intestinal section posteriorly. 
The digestive section may be at once recognised by the bright 
yellow-brown colour of the secretion which it contains. This 
fluid is evacuated after a time when the animal is irritated, but 
no experiments were made to determine its physiological pro- 
perties. 
1 Balfour was of opinion that in these forms they might be an ancestral 
feature (Balfour, ‘Comp. Emb.,’ vol. ii, ‘“ Tunicata”’). 
