226 SUMMARY OF QUERENT RESEARCHES RELATING TO 



either from the cell-body or its nuclei ; in many cases from both. 

 The process may be a connecting process, directly uniting one cell 

 with another, or a plexiform process, when it breaks up in the nervous 

 plexus in the central part of the central nervous system, or a trunk- 

 process when it is directly continuous with a nerve-fibre. The large 

 cells found in the ganglionic masses are called triangular ; their 

 form is best seen in the HaliotidsB and TrochidsB; the two upper 

 processes of the cells are always directly united with cortically placed 

 smaller cells ; the lower one is either a plexiform or a trunk-process. 

 Another well-marked form of cell, which is especially found in the 

 Trochidse, is small, pyriform, and always unipolar. The giant-cells 

 which are seen in the Pulmonata and the Opisthobranchiata are, like 

 the extremely small cells, always wanting in the Ehipidoglossata. 

 Others, which are known as central cells, have no processes. The 

 nucleus is, when fresh, always rounded and never, as in the Pul- 

 monata, reniform in shape ; the nucleolus is also round, is coloured 

 very intensely, and is highly refractive ; it is very rare for more than 

 one nucleolus to be present. 



2. The connective tissue in and around the central nervous system t 

 the ganglionic cells of the central nervous system always have true 

 cell-membranes ; these form a thin layer, consisting of a homogeneous 

 membrane, in which there are scattered oval nuclei, surrounded by a 

 finely granular protoplasm. Between the protoplasmic particles 

 there is a yellowish-brown pigment, which, like that of the ganglionic 

 cells, is extracted by alcohol. In satisfactory examples it is possible 

 to see that the membrane is a saccular process of the nerve-covering, 

 in which the cell lies embedded. The author comes to the conclusion 

 that the nervous investment forms a single envelope around the whole 

 central nervous system, which is continued from the neurilemma. It 

 is attached to the central system and serves as a certain support for 

 it, inasmuch as it sends processes into the nervous tissue. This 

 relation of the connective to the nervous tissue is regarded as being 

 a primitive one, uncomplicated by the relations which obtain in 

 higher forms. 



After describing the differences which obtain in different forms, 

 the author remarks that the tissue has a different structure in various 

 parts of the central nervous system of one and the same species ; and 

 he looks upon this as enforcing the views of Brock as to the great 

 variability of the connective tissue of molluscs. 



3. The central nerve-plexus : the author here enters upon a close 

 inquiry into the views of preceding writers,^ and concludes that in 

 the nuclear portion of the central nervous system of the Ehipido- 

 glossata we find neither the so-called dotted substance, nor neuroglia, 

 but that the whole is filled by a delicate (" subtile ") nervous plexus, 

 which has its origin in the ganglionic cells, 



4. Topography of the pedal cords, and the origin of their nerves : 

 in all the forms examined the pedal cord of either side was seen to 

 have a lateral groove which extends throughout its whole length ; it 

 is shallow in Fissurella, deep in the Haliotidso and Trochidas ; by the 

 aid of this groove each cord may be divided into an upper and a 



