Opisthobranchs from Laguna Beach
The determinations are by Dr. F. M. MacFarland
TECTIBRANCHS
Pleurobranchæa californica MacF. Only one specimen has been obtained at Laguna Beach, from a depth of from 15 to 20 fathoms. The specimen was mottled dark above and about 5 inches long. Dr. MacFarland informs me that this species is quite common in Monterey Bay and ranges much larger, almost up to 10 inches in length.
Navanax inermis Cooper. Black, yellow lines, blue spots, yellow edges. About two inches in length. Another specimen possibly may be the same species, black with yellow spots. Apparently the same form occurs at Balboa.
Aglaja (Doridium) purpureum Berg.? Brown, dredged 10 to 15 f.
NUDIBRANCHS
Triopha sp. Large, brown. Holdfast.
Flabellina iodinea Cooper. Narrow blue body, red appendages. Swims by lateral movements of the body. This beautiful nudibranch was first found near Laguna by Miss M. Cate, not far from Dana's point in 1916. In Jan. 15, '18, Mrs. May found a number near Laguna Beach.
Dirona picta MacF. Light brown, long thick appendages. Holdfasts and tidepools common in 1915.
Aegires sp. Knobs. Brick red, body clear.
Chromodoris universitatis Cock. Blue, yellow spots.
Polycera atra MacF. Red-brown, black stripes, brown spots. July 10, 1915.
Facelina sp. Body clear, appendages dark.
Ancula pacifica MacF.? Clear white, two yellow lines in front, one behind. Front appendages and two lateral tipped with yellow.
Cadlina Sp.? Dark brown, flattened.
Aeolidia sp. White to pink, appendages brown.
W. A. H.
Central Nervous System of the Sand
Dollar Dendraster Excentricus Esh
WILLIAM A. HILTON
There seems to be little or no literature on the central nervous system of this form of echinoderm. As might be expected, the general arrangement of radial and circumoral bands are much as in sea-urchins, such as shown especially by Delage and Herouard 1903. There are however some interesting features which make the study of this type of special value.
In this paper only the chief mass of the central nervous system is considered. The more evident parts of the central nervous system are arranged in general as in other forms. The circumoral nerves issue from under the lantern and run along the oral, cross over at the edge of the shell and then run along the aboral side. The five radial nerves converge at the five ocular areas near the center of the aboral region. The circumoral nerve ring is looped over and under parts of the lantern. Fig. [1] shows a part of the lantern and parts of three loops of the circumoral nerve trunk. In the center of the figure one fifth of the lantern is drawn in and from under it a radial nerve is shown in the lower part of the figure. To the left and to the right of the central bony part of the lantern the union of a radial with a circumoral nerve is shown. At the junction of each radial nerve with the circumoral, is a little thickening which seems to be a special cellular mass such as I have not found in other forms. Fig. [7] is a section through a part of a circumoral strand, much enlarged. There are only a few nerve cells, from one to two layers.
As the radial nerves leave the lantern they are quite evident in dissected specimens as they are close to the bony skeleton with very little connective or other tissues to obscure them. The use of aqueous methylene blue aids in following the smaller branches. Near the lantern the branches are small as shown in fig. [2]. When the region is reached where the upper and lower surfaces of the shell begin to fuse, the branches become larger and more irregularly arranged, as shown in the lower part of fig. [1] and fig. [2]. After the nerve turns to run on the aboral side there is no change in arrangement until the region of the tube feet is reached. In the region of the tube feet the nerves become more numerous, smaller and more regular. The general distribution of the nerves and the arrangement of the tube feet nerves are shown in fig. [4] which is from part of the upper end of the aboral nerve. The holes in the skeleton for the tube feet are shown as circles on each side of the diagram.
The general structure of the chief central nerve trunks is quite similar as shown in sections. Figs. [6], [7] and [8]. The nerve trunks have about one to two layers of cells, the main part of the nerves are composed of longitudinal fibers. There are not so many evident vertical fibers from cells as found in starfish and some other forms. This change in position of the fibers may be in part due to the general modification of structure. Whether this arrangement leads to other types of nerve association is a question.
When the nerve trunks are removed, stained in methylene blue and examined with the microscope something of the arrangement of the cells may be seen. In the circumoral and oral radial nerves the nerve cells are thickly massed from side to side, but in the upper part of the aboral nerve there is an evident arrangement of nerve cells in zones. There is usually a central more or less clear zone, next on each side a rather dense cell area and next on each side a very dense cell area, then a narrow nearly clear zone on each side again.
As a rule slightly larger cells are found near the nerve trunks and as some of these seem to send long branches out into the lateral trunks, they may be motor or sensory, the association neurones are probably the smaller cells in farther. The cells seem multipolar in most cases and in fact much more modified than the cells of starfish or sea-urchin. Figs. [9] and [10].
REFERENCES
(Contribution from the Zoological Laboratory of Pomona College)
Explanation of Figures
Fig. 1. Diagram of one fifth of Aristotle's lantern of Dendraster showing three loops of the circumoral nerve ring, and parts of three radial nerves, the central one partly hidden at its origin by the lantern. The nerves are in black. ×9.
Fig. 2. Drawing of part of the first part of an oral radial nerve. ×9.
Fig. 3. Drawing of the lower end of an oral radial nerve. ×9.
Fig. 4. Drawing of the upper part of an aboral radial nerve. The eye spot region is up in the figure. ×9.
Fig. 5. Camera lucida drawing of a part of an aboral nerve showing position of cell areas. ×70.
Fig. 6. Drawing of a section of an oral radial nerve. ×300.
Fig. 7. Drawing of a section of circumoral nerve. ×300.
Fig. 8. Drawing of a section of aboral nerve. ×300.
Fig. 9. Nerve cells from central regions of a radial nerve. The arrangement is as shown in the drawing, cells of various levels shown as one layer. Some of the processes possibly relate nearby cells, but most fibers run into the general fibrous mass. All fibers or fibrils are small. There is some indication of tigroid substance in some of the cells. ×450.
Fig. 10. Nerve cells from near a lateral branch from the radial band. ×450.
