90 THE VOYAGE OF H.M.S. CHALLENGER. 



One peculiarity, finally, deserving special mention is the presence in the majority of 

 well-preserved specimens of Hoplonemertea of a medio-dorsal longitudinal nerve homo- 

 logous to the medullary nerve described above (cf. p. 81, and PL XL fig. 8). Its connection 

 with the rest of the nervous system could not be satisfactorily made out, although traces 

 of a connection with the dorsal brain commissure were not wanting in many specimens. 

 Its presence is, however, significant, and its retention in the Hoplonemertea, where the 

 arrangement of the nerve-system has so considerably deviated from the primitive Palseo- 

 nemertea and Schizonemertea, must prevent us from underrating its morphological 

 significance. This will be more fully entered into in the chapter of General Considera- 

 tions at the end of this Report. 



SENSE-ORGANS, ACCESSORY GLANDULAR STRUCTURES, AND ORGANS 



OF UNKNOWN SIGNIFICANCE. 



The most conspicuous sense-organs of the Nemertea are without doubt the eyes. 

 Although eyes are absent in very many genera and species, and although in some species 

 pigment spots at the tip of the snout are regarded as such, other genera have very well- 

 marked and numerous eyes, with a hyaline hemispherical refractive body, a layer of visual 

 rods, and an optic nerve connecting the eye with the brain-lobes. These more highly- 

 developed eyes were previously known to occur in the Hoplonemertea, and the Challenger 

 material has confirmed these general conclusions. The most primitive of the Palaeo- 

 nemertea, Carinina, is not provided with eyes. Nor do I find traces of eyes in those 

 species of Eupolia which were contained in the collection, and of which the head was 

 studied in sections. Eupolia giardii is among these. However, it is known from 

 other researches (VTl) that different species of Eupolia have often very numerous eyes, 

 increasing in number with the growth and the age of the animal, and, moreover, that 

 these eyes resemble those of the Hoplonemertea in many respects. Nor were the Schizo- 

 nemertea of the Challenger provided with eyes that revealed their presence in the 

 microscopic sections, although I would not venture to affirm the total absence of eye- 

 like structures or pigment spots. In this respect the fresh animal often shows at 

 a glance what is very difficult to demonstrate in the series of sections, e.g., the 

 number and disposition of the eyes or pigment spots. As, moreover, these data can 

 hardly be of any taxonomic value for the determination of the Schizonemertea, I think 

 I may pass on to the discussion of the eyes of the Challenger Hoplonemertea. 



They have the characteristic histological arrangement already described and figured 

 by myself in a former publication on the subject (IX, fig. 42). A posterior layer of rod- 

 like elements, which is in direct connection with the optic nerve, is enclosed by pigment, 



