4 MORPHOLOGY OF 



Similar suckers^ occur as larval organs in Tunicata^ Ganoids, 

 and Amphibia. 



With regard to the meaning of the suckerj 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. 



The partial transparency of the body wall permits also an 

 indistinct view of the curious supporting rod of hypoblast 



1 Balfour was of opinion that in these forms they miglit be an ancestral 

 feature (Balfour, ' Comp. Emb.,' vol. ii, " Tunicata "). 



