ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
167 
Eyes and Central Nervous System of Salpa.* — Mr. M. M. Metcalf 
has been able to study the chain and solitary forms of eleven species 
of Salpa. In the solitary form of all species the eye has a horse-shoo 
shape ; this is not the case in any chain form, but each species has its 
own characteristic eye, all of which exhibit a fundamental conformity to 
a definite type ; in the course of their development the eyes of all tho 
chain-forms pass through a stage in which they are horse-shoe shaped. 
A subneural gland is always more or less well developed, and consists 
of two chambers beneath the brain, one on each side of the middle line, 
and each is connected with the peribranchial chamber by a comparatively 
large cylindrical duct. These structures appear to be homologous with 
the lateral ducts of Phallusia mammillata , which connect the subneural 
gland with the peribranchial chamber. The subneural gland is most 
highly developed in Salpa africana-maxima. 
In the course of its development the nervous system of Salpa passes 
through a stage in which it almost exactly resembles the nervous 
system of a nearly mature Doliolum ; in S. africana-maxima, which is, 
in this respect, the most primitive of the species studied, a remnant of 
this stage is retained in the adult. There is a solid wart-like antero- 
ventral protuberance from the ganglion ; a solid rod of cells is con- 
tinued forward from this protuberance, which soon fuses with the wall 
of the pharynx, and finally dwindles to a small hollow tube which can 
be traced to the ciliated funnel. 
The ganglion of Salpa is homologous with the visceral portion of 
the larval Ascidian nervous system ; in the embryonic condition there 
is an anterior thin- walled tube which opens to the ciliated funnel, and, 
later on, atrophies, and there is a posterior portion with a thickened 
ventral wall. The cells of the dorsal wall of the latter region proliferate 
to form the dorsal two-thirds of the ganglion, while the ventral third is 
formed by the thick ventral wall, which persists after the obliteration of 
the lumen of the neural tube. The cells corresponding to the sensory 
vesicle seen in the Tunicates lie in the thin-walled anterior portion, 
all of which atrophies. The ganglion of Salpa is, therefore, homologous 
with both the ganglion and the subneural gland of Ascidians (if, of 
course, Yan Beneden and Julin’s account of the development of the 
gland be correct) ; this homology is of importance as the eye of Salpa 
is formed from the ganglion. This eye cannot, therefore, be homologous 
with the eye of the Ascidian tadpole, as the latter is found in the sense- 
vesicle; nor can the eye of Salpa be homologous with the pineal or the 
lateral eyes of Vertebrates, as these are derived from the first primary 
brain-vesicle, while that of Salpa is formed from a secondarily acquired 
ganglion, derived from a more posterior portion of the nervous system, 
and one not represented in Vertebrates. 
The author calls attention to some points which militate against the 
recently expressed view of Prof. Biitschli f as to the probable phylo- 
genetic development of the lateral eyes of Vertebrates. Biitsehli’s 
description of the “ primitive eye ” does not correspond to the condition 
of any eye seen by Mr. Metcalf. The horse-shoe-shaped eye does not 
arise by the tripartition of a simple eye, but is distinctly horse-shoe- 
shaped from its first appearance. But the most important evidence is 
* Zool. Anzeig., xvi. (1893) pp. 6-10. f See this Journal, 1892, p. 775. 
