232 



COMPARATIVE ANATOMY 



CHAP. 



parts of the hypodermis, the so-called ciliated organs, ciliated clefts, 

 ciliated pits, ciliated prominences, which are regarded as olfactory 



FIG. 156. Section through a tactile 

 sensory organ of Macrobdella, after 

 Whitman, c, Cuticle ; ep, hypodermis ; 

 p, large clear cells ; gz, ganglion cells ; 

 n, nerve. The sensory cells are here 

 clearly seen to be long hypodermis cells, 



' FIG. 155.-Section through the eye the tactile hairs which they carry are 



of a land leech, c, Cuticle ; ep, hypo- not depicted. 



dermis ; p, large clear cells ; g, ganglion 



cells ; n,\ nerve ; dz, cutaneous gland 



cells ; pi, pigment (after Whitman). 



organs. The body epithelium at these points consists of ciliated sensory 

 cells, whose bases are prolonged as nerve fibres. These nerve fibres are 

 connected under the sensory epithelium with a plexus of ganglion cells, 

 which is itself again connected with the brain. In the Chcetopoda 

 the ciliated parts just mentioned are frequently depressed in the form 

 of pits or sacs, and are often protrusible. A special olfactory lobe 

 (ganglion olfactorium) may be developed on the brain in close proximity 

 to the ciliated organ, this lobe bearing the same relation to the 

 olfactory organ that the lobus opticus or ganglion opticum bears to 

 the eye. In the Nemertina these olfactory lobes are very strongly de- 

 veloped, and often sharply separated from the brain (cf. p. 216 and Figs. 

 140, 141). The ciliated organs here are pits which open outwardly by 

 means of longitudinal clefts at the sides of the head, and which in the 

 Schizonemertina are continued inwardly as ciliated canals, penetrating 

 to the interior of the cerebral olfactory lobes. It is probable that all 

 these ciliated organs are homologous formations, and correspond with 

 the ciliated grooves at the extremity of the head in the Turbellaria. 



In the Chcetopoda such organs have been found more or less developed in the 

 Capitellidce, Eunicidce, Nereidce, Phyllodocidce, Syllidce, Opheliacea, Typhloscolecidce, 



