POLYCHiETA. 



473 



tube is composed of numerous concentric laminae separated by inter- 

 vening rows of tolerably large-sized vesicles (fig. 335, b), while in 

 other forms the laminated tissue preponderates and the vesicular 

 tissue is imperfectly developed and is only present in parts of the 

 tube. 



As regards the geological range of Serptla, the earliest unequiv- 

 ocal types are found in the Carboniferous rocks, 1 but the genus has 

 been asserted to occur in still older deposits. In the Trias and 

 Lower Jurassic rocks Serpulce are not infrequent, but are usually 

 solitary in habit ; whereas social types of the genus are not uncom- 

 mon in the later Jurassic deposits, and entire beds of limestone in 

 the Lower Cretaceous rocks are sometimes made up of the tubes of 



till 



^ tip 



B 



Fig- 335- — Microscopic structure of the tube of some Tubicolar Annelides. A, Cross-section of 

 the tube of Scrpula sp. (Recent), showing laminated and tubulated calcareous tissue; b, Cross- 

 section of the tube of a Tertiary species of Serpida, showing laminated and vesicular tissue ; c, 

 Cross-section of the tube of Cornulites sp. (Silurian), showing an internal layer of laminated and 

 tubulated tissue, with an outer layer of a vesicular structure ; D, Oblique section of a tube of 

 Conchicolites gregarius (Ordovician). The figures show only parts of the sections, and are all 

 enlarged. (Original.) 



these Annelides. Numerous forms likewise occur in the Tertiary 

 rocks. 



In the genus Spirorbis (figs. 336-338), the tube is calcareous, and 

 is coiled into a flat spiral, one side of which is cemented to some 

 foreign body. The spiral may be either right-handed ("dextral") or 

 left-handed (" sinistral "), and the last volution is usually more or 

 less elevated, and is often extended into a free bent tube of no great 

 length. Internally the tube is usually open throughout, but in some 

 forms it is divided into chambers by a few calcareous partitions. 

 The tube is usually adorned externally with concentric striae or 

 annulations, sometimes with tubercles or spines ; and all the known 

 forms are of small size. In some species the tube is constantly 

 solitary, but in other cases great numbers are found together. The 



1 The Sei'piria parallela, M'Coy, of the Carboniferous rocks, as previously 

 shown, is really the siliceous "rope" of one of the Hexactinellicl Sponges 

 {Hyalostelia). 



