1 88 MANUAL OF ZOOLOGY. 



and with a short proboscis. There are thirteen pairs of bran- 

 chiae, placed on each side in the middle of the body. 



In the Nereides the body is greatly elongated, and consists 

 of a great number of similar segments, with rudimentary bran- 

 chiae. The head is distinct, and carries eyes and feelers, whilst 

 the mouth is furnished with a large proboscis, and often with 

 two horny jaws (fig. 57). In the Eunicea the branchiae are 

 usually well developed and of large size, and the mouth is 

 armed with seven, eight, or nine horny jaws. Eunice gigantea 

 attains sometimes a length of over four feet, and may consist 

 of more than four hundred rings. 



DISTRIBUTION OF ANNELIDA IN TIME. Of the Annelida the 

 only orders which are known to have left any traces of their 

 existence in past time are the Tubicola and the Errantia; of 

 which the former are known by their investing tubes, whilst 

 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, 

 is generally believed to belong to this order, but it is referred 

 by M. Barrande to the Pteropoda (Mollused). Cornulites and 

 Trachyderma, however, are undoubted Silurian Tubicola. The 

 Microconchus carbonarius is a little spiral Tubicolar Annelide, 

 nearly allied to the Spirorbis (fig. 56, b) of our seas, which is 

 not uncommonly found in strata belonging to the Carboni- 

 ferous 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. Oligochczta. Locomotion by means of rows of stiff bristles, 



or " setae ; " no foot-tubercles. 111. Gen. Lumbricus, Nals, 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, Terebetta, Sabella. 



