4 INTRODUCTION. 



in most of the cases above cited ; or they may be gregarious, and 

 either grow into massive bundles of parallel tubes, as in the coral 

 Favosites, the worm Galeolaria filiformis (Sow.), and the Bryozoan 

 Heteropora ; or again, they may be gregarious, but contorted, 

 forming dense tangled masses, as in the worm Galeolaria plexus 

 (Sow.), the Gasteropod Siliquaria, the Lamellibranch Teredina. 

 Similarly the structure of the tube, as a rule, does not give any 

 clue to the systematic position of the animal that made it. The 

 shells are often formed of miscellaneous fragments of foreign, 

 material, as in the sandy tubes of the Foraminifer Hyperammina, 

 of the rotifer Melicerta, or of the worm Sabella ; or as in the muddy 

 pipes of the Hydroid Tubularia or the worm Terebella ; or, again, 

 as in the short cylinders of sticks, shells, or fish scales, which 

 protect the Iarva3 of such insects as the caddis-worm. Or the 

 shells may be secreted by the external wall of the animal, as in 

 the case of most of those worms, Bryozoa, and Mollusca, which 

 have adopted the habit of dwelling in tubes. 



Moreover, when the tubes become specialized and more complex, 

 the same modifications appear in those of different animal groups. 

 Thus, when tubes which are long and narrow, still further 

 lengthen, the older part is left empty, and is cut off by the 

 formation of horizontal plates across the tubes. These plates are 

 found alike in the tubular shells of the corals, Bryozoa, worms, and 

 Mollusca ; they are known as tabulaB and diaphragms respectively 

 in the two first, and as septa in the two last. When, on the other 

 hand, the tubes are short and broad, the walls are strengthened by 

 the formation of loose, spongy tissue : this is found in such corals 

 as Diphyphyllum, in which it is called "vesicular endotheca"; in 

 such Bryozoa as Prasopora, in which it is said to be made up of 

 cystiphragms ; or in such worms as Cornulites, and such Brachio- 

 pods as Cyrtina, in which it is simply described as "vesicular 

 tissue." 



The fact that animals which belong to very different groups 

 have skeletons, which are based on the same plan, are built of 

 similar materials, possess the same parts, and vary within the same 

 limits, is of great interest to the zoologist. Excluding a few forms 

 which bore through floating wood, most of the tube-dwelling 

 animals live in shallow waters exposed to the scour of powerful 

 currents and tides, and therefore needing shelter from the drift of 

 sand and shingle. Hence they either occur imbedded in sand or 



