PART I. 



INTRODUCTION. 



1. THE PROBLEM OF TUBULAR FOSSILS. 



of the most familiar ways in which aquatic animals seek 

 protection from their enemies, is by surrounding themselves with 

 a tube, formed either of foreign particles held together by slime, 

 or of material secreted by the external surface of the body. As 

 these tubular shells are the most easily acquired of all effective 

 methods of defence, they have been adopted independently by 

 animals occupying very different zoological positions, and they 

 vary in complexity from simple tubes, to elaborate, specialized 

 skeletons. Examples of shells formed of primitive open tubes 

 occur in most of the phyla of the animal kingdom. Thus, in 

 the Protozoa there is the Foraminifer Syringammina ; in the 

 Coelenterata there are the Hydroid Tubularia and the coral 

 Cladochonus ; among the worms we find the order Tubicola and 

 the tube-building rotifers such as Melicerta ; and among the Bryozoa 

 there is the order Cyclostomata. The same type of skeleton is 

 developed by some of the Mollusca, such as the Gasteropod 

 Ccecum, the Scaphopod Dentalium, the Lamellibranch Teredo, 

 the so-called Pteropod Conularia, and the earliest Cephalopods ; 

 and it even occurs among the jointed Arthropods, as in the case 

 of the larval caddis-fly. 



Not only, however, is a skeleton consisting of a simple open 

 tube found in most groups of the Invertebrata, but the variations 

 of this tube, both in arrangement and structure, are similar in 

 different groups. Thus, the tubes may be single and straight, as 



