674 THE ANIMAL KINGDOM. 



* Planarians,' von Graff, Encyclopaedia Britannica (ed. ix.) xix. 1885. 



Rhabdocoelida, von Graff, Monographic der Turbellarien, Leipzig, 1882; cf. 

 Moseley, Nature, xxvii. Convoluta, Yves Delage, Etudes, &c., A. Z. Expt. (2), iv. 

 1886. Mesostomum, Jaworoski, Z. A. ix. 1886. Derostomum Benedeni, Francotte, 

 Bull. Ac. Roy. Belg. (3), vi. 1883. Graffilla, Bohmig, Z. W. Z. xliii. 1886. Ex- 

 cretory system of Micros toma, Zacharias, Z. A. viii. 1885. Development of Acoela, 

 Pereyaslawzew and Repiachoff, Z. A. viii. 1885. 



Tridadida : Gunda segmentata, Lang, Mitth. Zool. Stat. Naples, iii. 1882; of 

 freshwaters, lijima, Z. W. Z. xl. 1884 ; of N. American freshwaters, Silliman, Z. W. Z. 

 xli. 1885. Land Planarians, Moseley, Ph. Tr. 164, 1874; Id. Q. J. M. xvii. 1877 ; 

 Kennel, Arb. Zool. Zoot. Inst. Wurzburg, v. 1879. Transverse fission, Zacharias, 

 Z. W. Z. xliii. 1886, p. 271. Nervous system, Lang, op. cit. (supra). 



Excretory system of Rhabdocoela and Dendrocoelida (Triclads), Francotte, 

 Archives de Biologic, ii. 1881. 



Polycladida, Lang, Fauna und Flora des Golfes von Neapel, xi. Leipzig, 1884. 



For Rhodope Veranii Sidonia elegans, which appears to be a Turbellarian, 

 but was formerly referred to the non-palliate Gastropoda, see von Graff, M. J. viii. 

 1882; Bergh, Z. A. v. 1882. 



CLASS CHAETOGNATHA. 



This class of Vermes contains two genera, Sagitta and Spadella ; pelagic 

 with the exception of Sp. cephaloptera, which is littoral. They occur in all 

 seas. The organism is divided by two septa into three regions, a head, 

 body, and tail. The head has a slit-like mouth on its ventral aspect, is 

 guarded by a hood-like integumental fold on each side, and bears one or 

 two rows of stout spines, and a single row of sickle-shaped setae, or jaws. 

 The body and the tail carry one (Spadella} or two (Sagitta) pairs of lateral 

 fins, and the tail always terminates with a caudal fin. The body is covered 

 by a many-layered squamous ectoderm with an underlying delicate sup- 

 porting lamina. The fins contain a gelatinous skeletal lamella, on each 

 surface of which lie fine rods extending outwards from the body beneath 

 the ectoderm. The muscles are striated, broken up into special bundles 

 in the head, but in the body and trunk arranged in four bands, two dorsal 

 and two ventral. 



The nervous system consists of a supra-oesophageal ganglion in the 

 head, connected by long commissures to an infra-oesophageal ganglion 

 which lies near the centre of the body. They give off various nerves ; the 

 latter especially a very large number, which eventually break up into a 

 fine plexus with nodal ganglion cells. These parts lie in the ectoderm. 

 But two pairs of small ganglia are connected with the supra-oesophageal 

 ganglion, and lie in the mesoderm supplying the muscles of the head. 

 The organs of special sense are : a pair of eyes lying in the ectoderm 

 connected with nerves arising from the supra-oesophageal ganglion ; a 



