70 



THE VOYAGE OF H.M.S. CHALLENGER. 



cxterually as a small dentiform projection, and fringed at the outer edge with a row of 

 ciliated bristles. Both terminal plates are very slender, the outer, however, being some- 

 what broader than the inner, and of an oblong-linear form, with the outer edge naked 

 and straight, and terminating as a very small dentiform process, the apex narrowly 

 truncate, and, in common with the whole of the inner edge, having a row of long, ciliated 

 bristles. The inner plate is exceedingly narrow, conically tapered, and fringed through- 

 out with similar bristles. The length of the plates with respect to each other, as also to 

 the telson, varies somewhat in the different species, thus affording a comparatively good 



specific criterion. 



Luminous Globules.— These peculiar organs, for which I would suggest the above 

 designation, have not escaped the observation of earlier zoologists, and in part have even 

 been closely examined and described, most accurately by Professor Glaus.' In living 

 examples they are very conspicuous, and also in spirit-preserved specimens they admit of 

 being readily detected on dissection. The view generally fiivoured regarding the function 

 of these organs is, as is well known, that in some way they are subservient to sight, 

 whence they have been named by most authors " accessory eyes." I have carefully 

 examined these organs both in spirit specimens and in the living animal, and have 

 been led to form a very different opinion, conceiving them to have nothing whatever to 

 do with sight, but merely representing highly differentiated luminous organs. The 

 reasons on which I base such an assumption will be set forth in the sequel. I shall first 

 describe the organs under consideration as they occur in the genus Euphausia. 



In all the species of this genus, as also in most other Euphausiidse, they appear as 

 small globules, very conspicuous in the living animal by reason of their beautiful red 

 pioment and glistening lustre, and are symmetrically arranged both on the anterior and the 

 posterior divisions of the body. On the trunk are observed (see PI. XL figs. 1,2) two 

 pairs of such globules, the one situated within the coxal joint of the first pair of legs (see 

 PI. XII. fig. 16), the other within a corresponding dilatation issuing from the l)ase of 

 the penultimate pair of gills (see PI. XL fig. 9). On the tail they occur along the 

 ventral face, in the median line, between the liases of the pleopoda, each of the four 

 anterior segments having a single globule. In addition to the above mentioned globules 

 may be observed, as stated above, within the pedicle of the eyes, an organ of somewhat 

 similar appearance, though less completely developed (see figs. 7, 8). With tlie exception 

 of this last organ, all the others would seem to exhil)it precisely the same structure. 

 Those most easy to examine without dissection arc the hindmost pair on the trunk 

 (PI. XL fig. 10), as they lie in a position altogether external, immediately beneath the 

 inferior margin of the carapace. On placing the living animal under the microscope, 

 and applying a slight pressure l)y means of a cover-glass, in order to arrest its move- 

 ments, these organs admit of being examined through a comparatively jiowerful 



1 Ueber einige Schizopoden und iiiedere Malacostraken Messina's, Zeitschr. f. idss. ZooL, Bd. xiii. ])]>. 440, 447, 186,3. 



