224 PEOF. HXrXLEY ON THE 



The eye-spots of Amphioxus were single in all tlie specimens I 

 have examined ; in the very young Atmnocoetes, described by 

 Schulze, there are two such pigment-spots, separated by the very 

 short representatives of the cerebral hemispheres and olfactory 

 lobes. This suggests that the eye, like the nose, was primitively 

 simple in the Yertebrata, and that it has become divided in 

 the same way as the nose. In this case the involution of the 

 epiblast, out of whichi the cornea and the crystalline lens are 

 developed, should liave been primitively a median sac ; and it is 

 a curious circumstance that, in the very young tadpole, Mr. W. 

 K. Parker, E.E.S., has described and figured a transverse groove 

 connecting the eye- sacs. 



I am unable to find any thing in the structure or mode of deve- 

 lopment of the Marsipobranchii which gives this group more 

 than an ordinal value in the class Pisces. Their great peculi- 

 arities are the structure of the skull, the presence of a naso- 

 palatine passage which opens posteriorly in the Myxinoids, and 

 the existence of a large superior median brain-lobe. 



As respects the first point, the skull is strictly comparable 

 with that of the embryo of any higher Yertebrate, being com- 

 posed of a parachordal occipital portion, of largely developed 

 trabeculae, and of auditory capsules. In the Lampreys the carti- 

 laginous hyoidean and mandibular arches are represented, and 

 the curious facial cartilages appear to me to be reducible to the 

 type of the labial cartilages of the Elasmobranchs. The deve- 

 lopment of the olfactory organ of the Lamprey proves that the 

 single nasal sac of Amphioxus is the homologue of the nasal sac 

 of the Marsipobranchii (at least of that part which is lined by 

 the Schneiderian membrane), to which, however, two olfactory 

 nerves, produced apparently by the division of a primitively 

 simple and median nerve, proceed. The term " Monorhina," 

 applied by Haeckel to the Marsipobranchii, therefore, is not 

 strictly applicable, and I cannot attach any great taxonomic 

 value to the structure of the olfactory organs in this group. 

 The external duplication of the nasal apertures in the higher 

 Yertebrata appears to me to be chiefly due to the fact that, 

 in them, the cerebral hemispheres are thrown out in front of 

 the anterior cerebral vesicle, the front wall of which (the lamina 

 terminalis of the third ventricle of the fully developed brain) 

 corresponds with the anterior end of the cerebro-spiiial axis of 

 Amjphioxus, and attains a large size and considerable downward 



