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But as soon as the outer and inner walls of the tube, as in 

 the Pycuogonida, are hard and chitinised, the problem assumes 

 a quite different aspect: Longitudinal muscles are no longer met 

 with, because the unelastic outer wall would no longer allow any 

 contraction. This wall has now become a firm support and serves 

 as a starting point for the muscles that are attached to the inner 

 wall. If the latter were also circular, in transverse section, and 

 at the sarne time consisted of hard chitine, all expansion and 

 contraction would become impossible. Such a case would, of course, 

 be inconceivable. The inner wall must therefore be flexible. 



The first case to assume, in which the lumen of the tube, 

 under the circumstances, can be expanded, is that there are firm 

 points in two places, the two chitine halves to be further bent 

 by muscles. The elasticity of the walls alone is then able to cause 

 a narrowing, but even circular muscles can be useful here in some 

 cases. It stands to reason, however, that, so far, there can be 

 only a minimum of mobility. Nor do I think a suctorial organ 

 arranged like that, will be found in many kinds of animals. Yet 

 the pharynx of true spiders gives the best idea of it (tig. 1). On 

 a transverse section we find only 2 dilatators, which can expand 

 a little the sides of the flattened tube, so that this apparatns 

 cannot be a very powerful means of suction. But if I draw 

 attention, in this instance, to the existence of a strong suctorial 

 organ in these animals, this imperfect development of the pharynx 

 is more easily accounted for. This organ, which is found close 

 to the thin oesophagus after the latter has passed through the 

 central nerve-system, does, however, not belong to the parts that 

 have developed out of the stomodaeum, and can therefore not be taken 

 into consideration in making a comparison with the proboscis. 



The next case would be the existence of three fixeü points in 

 the inner chitine coating. Now it is feasible to effect a consi- 

 derable widening of the tube by means of three muscles, nor does 

 such an arraugement belong to the rare occurrences. 



We must, however, at once distinguish two different cases: 



1° the organ is wanted for suction only, or 



