CARADOCIAN CYSTIDEA FROM GIRVAN. 493 



latter, moreover, the reduction of the pectinirhombs has been carried so far that rhomb 

 1-5 has entirely disappeared. Another fact indicating a decrease of function in the 

 pectinirhombs is their partial closing by epistereom, as in P. gibba. 



^ 556. This obsolescence of the pectinirhombs may be compared with the dis- 

 appearance of certain rhombs and the disjunct structure of others in some species of 

 C/ieirocrinus. It is probably connected with the increased size and activities of the 

 cut ; and though the correlation with the reduction in size of periproctals is not exact, 

 it is at least suggestive that the longest and best developed pectinirhombs occur in the 

 species with the smallest periproct and largest periproctals, viz, P. jilitexta. As to 

 what those activities of the gut were, we can but speculate. One assumes that the 

 pectinirhombs were for the purpose of aeration or respiration. If they decreased, that 

 function would be thrown on some other organs. The arms, so far as one can see, 

 retain the same character throughout the genus, and at no time can they have rendered 

 much aid in this respect. Thus one concludes that the function was taken over by the 

 gut and effected per anum. 



§ 557. The question still remains : did the decrease of the pectinirhombs indirectly 

 cause the increase of the gut ; or did the increased size and activity of the gut directly 

 cause the decrease of the pectinirhombs by pressure on their inner surfaces ? Con- 

 sideration of the peculiar distribution of the pectinirhombs in the earlier Glyptocystidea 

 led mc, it may be remembered, to the view that their absence along certain tracts was 

 due to the pressure of the gut-coils on the inner wall of those tracts. Following up 

 this idea, I supposed at first that the atrophy of the pectinirhombs was due directly to 

 the hypertrophy of the gut. But this conclusion is opposed by the fact that the 

 rhomb-folds are, so far as one can see, better developed on the inner face than on the 

 outer, and especially by the closing of the folds in some cases on their outer face. 

 Clearly, then, the increase of the gut was not the cause in this case, whatever it may 

 have effected at an early stage of the phylogeny. The answer, then, is that the decrease 

 of the rhombs caused the increase of the gut. 



§ 558. To use the word " cause " is perhaps to outstrip our evidence. In any case 

 the supposed action was indirect ; but there is also the possibility that both phenomena 

 were the independent result of some other cause. It is easy enough to explain an 

 increase in the size of the gut ; for explanations, whether right or wrong, abound. But 

 it is not so easy to see why such highly-developed structures as pectinirhombs should 

 ever have been diminished in size, still less why any should have utterly vanished. 

 Some modification of their outline was doubtless due to the change in shape of the 

 theca, but that was never enough to account for the extremes of P. Rugeri and 

 P. procera. We must look, therefore, for some external cause. Respiratory organs of 

 this kind would be hindered in their work by any foreign body covering or clogging 

 the slits. Was there any difference in conditions that would lead us to suppose any 

 such foreign body in the case of the British species and not of the American ? Certainly 

 there was, and a great difference. The American fossils are all preserved in their 



