2l6 MANUAL OF ZOOLOGY. 



the latter are only recognised by the tracks which they left 

 upon ancient sea-bottoms, or by their burrows in sand or mud. 

 These tracks and burrows of Annelides are found commonly in 

 rocks of almost all ages from the Cambrian period upwards. 

 Those tracks which have been caused simply by the passage 

 of the worm over the surface of the mud are termed by Mr 

 Salter Helminthites, whilst the burrows are called Scolites (or 

 Scolithus). 



Tubicolar Annelides are known to occur from the Silurian 

 Rocks upwards. The well-known Silurian fossil, Tentaculites, 

 has been often referred to the Tubifola^\3n\. is almost certainly 

 Pteropodous. ComulitfS, Serpulites, Ortonia, Trachy derma, 

 Spirorbis, and Conchicolites are, however, genuine Silurian 

 Tubicola. The Microconchus carbonarius is a little spiral 

 Tubicolar Annelide, nearly allied to the Spirorbis (fig. 79, /;) 

 of our seas, which is not uncommonly found in strata belong- 

 ing to the Carboniferous period ; and the genus Spirorbis 

 itself is represented even in the Silurian period. 



TABULAR VIEW OF THE ANNELIDA. 



Division A. ABRANCHIATA. No external organs of respiration. 



Order I. Hirudinea. No bristles or foot- tubercles : locomotion by 



means of a suctorial disc at one or both extremities. 111. Gen. 



Hirudo, Clepsine, Pontobdella. 

 Order II. Oligochata. Locomotion by means of rows of stiff bristles, 



or "setae ;" no foot-tubercles. 111. Gen. Lumbricus, JVais, Tubifex. 



Division B. BRANCHIATA. Respiratory organs in the form of external 

 branchiae. 



Order III. Tubicola. Body protected by a calcareous or arenaceous 

 tube. Branchiae attached to, or near, the head (Cephalobranchiata}. 

 111. Gen. Serpula, Terebella, Sabella. 



Order IV. Errantia. Animal free, with setigerous foot-tubercles. 

 Branchiae in tufts, attached on the sides of the body, in the middle 

 of dorsal region only, or along its entire length (Dorsibranchiatd). 

 111. Gen. Arenicola (Lob-worm), Nereis (Sea-centipede), Aphrodite 

 (Sea-mouse). 



CLASS III. CH^TOGNATHA (Huxley). The remaining class 

 of the Anarthropoda has been recently constituted by Professor 

 Huxley tinder the name of Chcetognatha, for the reception of 

 the single genus Sagitta, which had been formerly placed 

 amongst the Annelida. By Professor Rolleston, however, the 

 Chcetognatha are placed in the division Nematelmia of the 

 Annuloida, in the immediate neighbourhood of the Nema- 

 toidea. 



The Sagitta are singular marine animals, transparent, and 

 elongated in form, and usually not more than an inch in 



