; 7 
and the subneural gland and to the metamorphosis in the nervous 
| system by which the adult condition is reached. I published in the 
Johns Hopkins University circulars for April 1891 a brief preliminary 
note of some of my results. I which here to call further attention to 
a few facts and offer a few theoretical considerations. 
In the solitary form of all species of Salpa the eye is horse-shoe 
shaped. The chain forms of no two species of Salpa have the same 
shaped eye, but each species has its own characteristic eye. I will not 
attempt without figures to describe these most decided variations. The 
eyes of the chain forms, though so diverse, still show a fundamental 
conformity to a definite type. For this reason the study of the eyes 
gives good evidence as to the nearness of relationship between species. 
Salpa pinnata and Salpa Chamissonis, Brooks (n. sp.), each have 
five eyes in the chain form: one, large, on the dorsal side of the brain 
(the only one known previous to my studies); the other four, smaller 
and arranged in pairs, one pair on the antero-dorsal face of the gan- 
glion, the other pair in the middle of its posterior face. One or both 
pairs of the smaller eyes are usually represented in the chain indivi- 
duals of other species, being often, however, in a somewhat degene- 
rate condition. The large dorsal eye is a complex structure. In its 
anterior half the rod cells are ventral and the pigment cells dorsal : 
in its posterior half this arrangement is reversed. The optic nerve 
arises from the non-cellular core of the ganglion and passes up over 
the dorsal surface of the posterior part of the eye, innervating there 
the dorsally lying rod cells: then it pushes ventralwards through the 
eye to innervate the ventrally lying cells of its anterior portion. 
The larger dorsal eye of the chain form of S. runcinata- fusi- 
formis, 8. Africana-maxima, S. cylindrica, S. hexagona, S. costata- 
Tilleni , S. cordiformis-zonaria, S. democratica-mucronata, S. pin- 
nata, and S. Chamissonis, develops as a disc-shaped plate of cells 
pushed up from the dorsal surface of the ganglion toward the ecto- 
derm. The fibres of the optic nerve rise from the dorsal part of the 
ganglion and enter directly the centre of the ventral face of the eye 
disc. In the course of its development this disc tips forward, suffering 
a reversal by which the originally anterior edge becomes posterior and 
vice versa, while its ventral surface becomes dorsal. By this same re- 
versal the optic nerve comes to lie along the dorsal surface of the poste- 
rior half of the eye. The corresponding eye of the chain S. scutigera- 
confederata and S. bicaudata does not suffer this reversal, indicating 
that these two species belong in a separate group. 
Previous to the assumption of the disc-like form the developing 
eye of the chain Salpa passes through a stage when it resembles in 
