796 RECORD OF CURRENT RESEARCHES RELATING TO 



arranged indefinitely, or, as in a small Sagitta observed by Langerhans, 

 tbey may be arranged by sixes in forty rings. The eyes are found on 

 tbe upper surface of the head, and form two blackish spots of so 

 small a size as to be indistinguishable to the naked eye. When mag- 

 nified, the eyes are seen to be formed by sj^heres, made uji of small 

 cells, which are enclosed by the usual transparent epidermis, and are 

 sharply separated off from that layer ; the part exposed to the light 

 is coloured by a blackish pigment and contains a transparent lens ; 

 these last two structures are surrounded by a circlet of numerous, 

 highly refractive rods ; the form of these rods is highly characteristic. 

 The end which is approximated to the pigment and lens is thin and 

 cut short along a transverse axis ; the rods then thicken somewhat, 

 but suddenly diminish in width towards their tip. Still further and 

 closer examination leads to the conclusion that the eye of the Chfeto- 

 gnatha is not a simple but a complex structure, aifording many points 

 of resemblance to the same organ in the Crustacea ; there are three 

 lenses, and it is clear that the eye is made up by the fusion of three 

 simple ocelli. The " optic epithelium " consists of a rod and a layer 

 of granules, which are sharply distinguished from one another. The 

 organ is comj)letely enclosed in the epidermis, and is invested on its 

 outer face by a thin layer of flattened ejiidermic cells. 



The olfactory organ does not seem to have been hitherto correctly 

 apprehended ; it is placed near the eye, on the upper surface of the 

 head, and behind the supra-oesophageal ganglion ; it is unpaired, and is 

 of a simple character. There is a delicate epithelial band, made up 

 of fine cylindrical cells lying on the trausjiarent cells of the epidermis, 

 and forming a slight projection. In the middle of the band there are 

 two or three rows of cells which are provided with very long delicate 

 cilia. The epithelial bands vary in form in various species ; they 

 are supplied by two well-developed nerves, which arise from the 

 posterior surface of the supra oesophageal ganglion, and pass to the 

 olfactory organ along a line parallel to that of the optic nerves. 



The nervous system of these creatures is of especial interest, inas- 

 much as the chief ganglia and the nerves from them are imbedded in 

 the epidermis, while some of the smaller cejDhalic ganglia, with their 

 nerves, belong to the mesoderm ; we have, therefore, in the nervous 

 system of the Chsetognatha to distinguish an ectodermal and a meso- 

 dermal portion. The former consists of two central organs, the 

 supra-oesophageal and the ventral ganglia, together with their nerves ; 

 the mesodermal portion is imbedded in the head. The supra- 

 cesophageal or cephalic ganglion is imbedded in the epidermis, and 

 forms a slight outwardly-projecting protuberance; in form it is 

 regularly hexagonal, and it is separated on its lower surface by a 

 supporting lamella from the subjacent musculature. It is made up 

 of a mass of delicate fibres and of small ganglion-cells; from 

 the former there are given off four stronger and six more delicate 

 nerves ; two of the former are the anterior motor nerves, and the 

 other two are the commissures by which these are connected with 

 the ventral ganglionic mass. The six more delicate nerves are all 

 sensory. The ventral is larger in size than the cephalic ganglion, 



