BIGELOW: THE CTENOPHORES. 371 



in other words these differences are no greater than may be found in 

 large series of pileus, which, as Moser has pointed out, shows great 

 individual variability. In fact, it would be impossible to pick out 

 this Mediterranean specimen from a series of pileus similarly pre- 

 served and of about the same size. 



We must remember that with preservation Ctenophores almost 

 always suffer more or less alteration of form and proportion, even 

 though they may be in good condition anatomically, and for this 

 reason slight differences in form and proportion between preserved 

 specimens are to be looked on with suspicion. This specimen, which 

 is the oldest Pleurobrachia yet described for the Mediterranean 

 would require but little additional development for it to attain the 

 stage so beautifully figured by L. Agassiz for pileus. All that would 

 be necessary is progressive growth of the aboral half of the body 

 resulting in proportionately longer stomach, ribs, and tentacular 

 sheaths, and bases. This is exactly the line of development in pileus; 

 and I have been able to follow it through successive steps to a speci- 

 men 24 mm. long. The most interesting feature of growth is the 

 increasing size of the tentacular sheaths. 



The progress of the change is already to be seen in the small Medi- 

 terranean specimens, and in larger pileus of 15-20 mm. The 

 sheaths keep pace with the ribs, finally reaching nearly to the oral 

 pole of the body. Moser ( : 09) has pointed out that the body, at first 

 nearly globular, becomes more and more cylindrical as it increases in 

 size, and that the stomach becomes proportionately longer, and that 

 the tentacle-bases grow proportionately larger, and lie nearer and 

 nearer the stomach. And my own studies entirely substantiate her 

 account. In the adult condition the tentacular bases and the gastric 

 cavity are variously affected by different preservatives. In life, so far 

 as I have seen, the sheaths are always at an appreciable distance 

 from the oesophageal canals and gastric wall; and the relative posi- 

 tions of the organs are retained fairly well in osmic-acid material. 

 But in all the available specimens preserved in formalin the gastric 

 cavity is so much dilated that its normal flattening is entirely obscured, 

 and it is in close contact with the tentacle-sheaths. This warns us 

 how cautiously we must employ such characters in preserved Cteno- 

 phores, especially in describing new species. The facts outlined above 

 show that there is no sound distinction between rhodopsis and pileus. 

 Instead of being a local species peculiar to the Mediterranean as 

 Moser supposed, the former is merely an intermediate stage in the 

 development of the latter, and therefore there is no longer any reason 



