ANNULOSA: CH^ETOGNATHA. 189 



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

 Branchise in tufts, attached to the sides of the body, in the middle 

 of dorsal region only, or along its entire length (Dorsibranchiata}. 

 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 under 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 

 Chcztognatha 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 

 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 

 hinder extremity. 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 vesicula seminales. The embryos are not 

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

 the Classification of Animals, p. 52.) 



