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MANUAL OF ZOOLOGY. 



oral aperture is furnished with unciform corneous bristles or 

 "falces," which act as jaws. The alimentary canal is straight, 

 and terminates in an anus placed at the base of the tail 

 below. 



"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 pig- 

 mented imperfect eyes 

 rest. The ovaries, sac- 

 cular 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 seminales. 

 The embryos are not ciliated, and undergo no metamorphosis " 

 (Huxley). 



The species of Sagitta are found, living in the open sea, in 

 the Mediterranean, and in the Atlantic and Pacific Oceans. 



Fig. 134. Morphology of Chcetognatha. A, Sag- 

 itta tricuspidata, of the natural size : o One of the 

 ovaries ; sp Orifice of one of male organs of re- 

 production. B, Head of the same, viewed from 

 beneath and greatly enlarged, showing the horny, 

 setiform jaws. (After Saville Kent.) 



LITERATURE. 



GEPHYREA. 



1. " History of British Starfishes and other Animals of the Class Echino- 



dermata." Edward Forbes. 1841. 



2. "Memoire sur 1'Echiure." Quatrefages. 'Annales Sci. Nat.' 1847. 



3. " Ueber Thalassema." Krohn. 'Arch, fur Anat. und Physiologic.' 



1842. 



4. ' ' Observationes anatomicae de vermibus quibusdam maritimis." 



Miiller. 1852. 



5. " Larven von Phascolosoma. " Selenka. 'Zeitschrift Wiss. Zool.' 



1875; 



o. " Beitrage zur anatomischen und systematischen Kenntniss der Sipun- 

 culiden." Keferstein. Zeitschr. fiir Wiss. Zool. ' 1865. 



