466 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
the Dentalia are representatives of ancestral forms might be accepted 
if the arms of the Cephalopoda can be shown to be the homologues of 
the tentacles of Dentalium , but it seems to Dr. Plate more probable that 
they are pedal in nature. Prof. Grobben urged in reply that the radula 
and pleural ganglia were probably possessed by the ancestor of the 
Lamellibranchiata, and had, therefore, no weight, while the arguments 
as to the pedal nature of the tentacles of Cephalopods were not conclu- 
sive. Prof. Leuckart assigned a position between the Gastropoda and 
Lamellibranchiata to the Solenoconcha. 
Molluscoida. 
o. Tunicata. 
Eyes and Subneural Gland in Salpa.* — Mr. M. M. Metcalf has a 
preliminary note on the results of his studies on Salpa. The eye of 
Salpa pinnata, solitary form, in every part faces toward the mid-dorsal 
point of the brain ; the retina is composed of three layers of cells. 
These are the pigment cells, equal in size to the unmodified nerve-cells ; 
a layer of cells which closely resembles the nerve-cells, and a layer of 
rod-cells. In a young embryo a dorsal ridge of cells becomes developed 
from the ganglion, and has, from the first, the characteristic horse-shoe 
shape. The ridge increases in size as the ganglion grows. When the 
central cells of the ganglion degenerate, the cells in the core of the 
ridge degenerate also. The retinal cells do not begin to assume their 
characteristic appearance till some time later. The most peripheral 
layer of cells over the portion of the ridge which is destined to form 
the retina become larger, and then columnar. The adult condition is 
reached by an increase in the size of the rod-cells, a greater thickening 
of the walls of their inner ends, and by a greater deposit of pigment 
granules in the posterior cells of the retina. 
The eye of Salpa 'pinnata , chain-form, agrees in structure with that 
of the solitary, but its shape is quite different ; in addition to the 
median dorsal eye there are also two pairs of much smaller eyes. One 
pair is placed on either side of the middle point of the posterior face of 
the brain, while the other pair lies on the dorsal surface, one on each 
side. While the eyes of solitary forms of various species show a close 
agreement with those of S. pinnata , those of the chain form show very 
marked differences in the different species ; differences which are 
constant and characteristic, and which Mr. Metcalf believes to have 
great value in a systematic study of the group. The early stages of 
the development of the eye of the chain- form of S. pinnata are almost 
the same as those of the solitary forms. The smaller eyes develope later 
than the larger eye, and are formed by the histological differentiation of 
certain of the ganglion cells situated in the positions where the smaller 
eyes are found in the adult. 
The Salpae differ from the Ascidise in having the cavity of the central 
nervous system, which is present only in early stages, continuous with 
the lumen of the ciliated funnel. In an early stage of the chain-form, 
long before any trace of the eye appears, the cavity of the brain, and 
the lumen of the funnel (or “ dorsal tubercle ”) open into each other by 
* John Hopkins Univ. Circ., xi. (1892) pp. 78-9. 
