ANNULOSA : CH^ETOGNATHA. 169 



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

 branchiae. 



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



txibe. Branchiae attached to, or near, the head (Cephalobranckiata). 



111. Gen. Serpula, Terebella, Sabella. 

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



Branchiae in tnfts, attached to the sides of the body, in the middle 



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



111. G-en. Arenicola (Lob-worm), Kereits (Sea-centipede), Aphrodite 



(Sea-mouse). 



CLASS III. CH^ETOGNATHA (Huxley). The remaining class ot 

 the Anarthropoda has been recently constituted by Professor 

 Huxley under the name of Ghcetognatha, for the reception of the 

 single genus Sagitta, which had been formerly placed amongst 

 the Annelida. 



The Sagittce 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 setse, 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 hinder extre- 

 mity. A single oval ganglion lies in the abdomen, and sends, 

 forwards and backwards, two pairs of lateral cords. The lateral 

 cords unite in front of and above the mouth into a hexagonal 

 ganglion. This gives off two branches which dilate at their 

 extremities into the spheroidal ganglia, on which the darkly 

 pigmented imperfect eyes rest. The ovaries, saccular organs, 

 lie on each side of the intestine and open on either side of the 

 vent ; receptacula seminis are present. Behind the anus, the 

 cavity of the tapering caudal part of the body is partitioned 

 into two compartments ; on the lateral parietes of these, cellular 

 masses are developed which become detached, and floating 

 freely in the compartment, develop into spermatozoa. These 

 escape by spout-like lateral ducts, the dilated bases of which 

 perform the part of vesiculce semincdes. The embryos are not 

 ciliated, and undergo no metamorphosis.' (See Introduction to 

 the Classification of Animals, p. 52.) 



