MORPHOLOGY AND HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM OF SUCCINEA PUTRIS (L.)

by

H.COOK (Departmentof Zoology,Free University,Amsterdam) 2

_ I. INTRODUCTION

The central nervous system (CNS) of pulmonates has, of late, regained much of the interest that it held around the turn of the century. Many investigators are employing the improved equipment and techniques which are available, to gain a better understanding of the central gan- glia of these gastropods. The present publication reports the results of an investigation into the morphology and histology of the CNS of the stylommatophoran Succinea putris (L.), after a short description of the population from which the animals studied were taken.

The Succineidae have been shown to be remarkable stylommatophorans because they have more properties in common with the Basommatopho- ra than other species of their Order: chromosome number, the struc- ture of the egg mass, and also to a limited degree the habitat. The CNS has also been shown to have certain special features in common with basommatophoran pulmonates. Some of these points will be discussed briefly. From the literature it is apparent that, in general, stylommatophoran species have haploid numbers above 20, while Basommatophora have lower chromosome numbers, if some polyploid species are excluded (PERROT, 1938; PERROT and PERROT, 1938; LE CALVEZ and CERTAIN, 1950; INABA, 1959; BURCH, 1960a, b, 1961, 1963, 1964a; BURCH and HEARD, 1962; BURCH and KiM, 1962; BURCH et al., 1964). A notable exception to this rule is formed by several species of the Succineidae. INABA (1945) and KOYAMA (1955), studying three Japanese species, found a haploid number of 17. BURCH (1964b) found the succineid snail Catinella Rotundata to have a chromosome number of 5 (2n = 10), the lowest found thus far for any mollusc, while Catinella (Mediappendix) vermeta has six pairs of chromosomes. S. putris, investigated by PERROT (1938) and .f. ovals, studied by HICKMAN ( 1931 ), however, were reported to have haploid numbers of 22 and 20 respectively. The possibility that the latter two species are polyploid has been suggested by some in- vestigators. Reports of stylommatophoran species in other families, with haploid numbers below 20 were shown to be in error or supposed to be unreliable (BURCH, 1960a; BURCH and KIM, 1962; BURCH and HEARD, 1962). BURCH (1964b) further suggests that the chromosome number is proportional to the degree of specialization. These facts sug- gest that the Succineidae are a family of Stylommatophora which is unique because of its low degree of complexity, and its likeness to basommatophoran snails. THIELE (1931) also places the Succineidae,