On the Structure of the Cells Bearing the Velar Cilia of the Nudibranch Veliger

THE object of the present paper is to extend the previous observations of the author (3) on the nervous control of the cilia of the Molluscan velum and to provide a description of the cytoplasmic structure of the cells which bear these cilia. It is hoped that a revision of this subject, based as far as possible on observations made on the living cell rather than on fixed material, may be of use in the study of the mechanism of ciliary activity from both the chemical and structural points of view. In order to increase the definiteness of the observations, they were restricted to one type of veliger. That of Aeolidia papillosa was chosen on account of the ease with which material of this species could be obtained. The veligers were studied at the stage at which they are still contained within the egg-capsule but can be seen actively moving within it. These movements may be observed for several days before the larvae escape, and during this period the structure of the velum alters very little, except by growth. By the time that the movements of the cilia are active, the cells of the velum are fully developed and the beat of the cilia has the form seen in the free-swimming veliger. The intermissions of the beat, described in the previous paper, already occur and the nerve-supply of the velum is complete. This stage, therefore, gives a convenient point at which the veligers can be readily obtained in bulk and the cells studied as fully effective organs. Various methods were used in the investigation of these cells. Most of the internal structure can be made out in the living cell by the use of vital stains and several of these were used. The cells were studied both in their natural position on the velum and free in the water after the veliger had been crushed. Such cells may often be seen to move through the water, driven by the beat of their cilia, which continues for some time. The ciliary mechanism has therefore not been damaged by this treatment. These observations were supported by study of the cells in fixed material, and for this purpose as many different methods as possible were used. For the study of the general structure of the cells, formalin, Flemming's chrom-osmic mixture with and without acetic acid and Carnoy's fluid were used as fixatives, and as stains haematoxylin and Mann's methyl blue-eosin mixture: for that of the chromatin,