The distribution of nerve cells inHydra attenuata Pall

Polyps ofHydra attenuata Pall. were subdivided into four axial fragments (I-IV) of about identical length. Ecto- and endoderm of each of these fragments were separated from each other and dissociated according to the technique of David (1973). Each preparation was qualitatively and quantitatively examined as to its content of nerve cells.The presence of 11 different types of nerve cells (Tardent and Weber, 1976) could be confirmed. While multipolars (M₁, M₂), symmetrical bipolars (B₁, B₂) and unipolars (U₁, U₂) were found in both layers, asymmetrical bipolars (B_3-7) are limited to the endoderm (Fig. 1), which, as a whole, contains fewer nervous elements than the ectoderm (Fig. 2). The ecto- and endoderm of the tentacles could not be separated from each other and the dissociation was only partly successful. It was, however, possible to ascertain that among the nerve cells present in the tentacles the most numerous are the symmetrical bipolars (B₁, B₂).