198 MANUAL OF ZOOLOGV. 



Traehyderma, however, are undoubted Silurian TuUcola. The 

 MkroconcJiMS carbonariiis is a little spiral Tubicolar Annelide, 

 nearly allied to the Spirorbis (fig. 58, b) of our sea^ 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 OS THE ANNELIDA. 



Division A. Abranchiata. — No external organs of respiration. 



Order!. 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. Oligochmta. — Locomotion by means of rows of stiff bristles, 



or " setiE ;" no foot-tubercles. Vii,Ge:a. LumbricuSjNaw, Tubifex. 



Division B. Branchiata. — Respiratory organs in the form of external 

 branchiae. 



Order\\\. Tubicola. — Body protected by a calcareous or arenaceous 



tube. Branchis attached to, or near, the head [Cephalobrdnchiatd). 



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), Aphrvdite 



(Sea-mouse). 



Class III. Ch^tognatha (Huxley). — The remaining class 

 of the Anarthropoda has been recently constituted by Professor 

 Huxley under the name of Chcstognatha, for the reception of 

 the single genus Sagitta, which had been formerly placed 

 amongst the Annelida. By Professor RoUeston, however, the 

 Chmtognatha axe. placed in the division Nematelmia of the 

 Annuloida, in the immediate neighbourhood of the Nema- 

 ioidea. 



The SagittcB are singular marine animals, transparent, and 

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

 length. The following are the characters ascribed to the. class 

 by Huxley : — 



"The head is provided with several, usually six, sets of 

 strong, bilaterally symmetrical oral setae, two of which, long 

 and claw-like, lie at the sides of the mouth ; while the other 

 four sets are short, and lie on that part of the snout which is 

 produced in front of the oral aperture. The posterior part of 

 the body is fringed on each side by a delicate striated fin-like 

 membrane, which seems to be an expansion of the cuticle. In 

 some species the body is beset with fine setae. The intestine is 

 a simple, straight tube, extending from the mouth to the anus ; 

 the latter opens on the ventral surface, just in front of the 



