Define eutyly.

Explanation

**Step-by-Step Solution:**

1. **Definition of Eutyly**: Eutyly refers to a biological phenomenon observed in certain organisms, particularly within the phylum Aschelminthes (or Nematoda), where the organism maintains a fixed number of somatic cells throughout its life.

2. **Characteristics of Eutyly**:
- **Fixed Number of Cells**: Once the organism reaches maturity, the total number of somatic cells does not increase. This means that the organism has a predetermined number of cells that it will have for its entire life.
- **Cell Size Variation**: Although the number of cells remains constant, the size of these cells can change. Cells may grow larger or shrink, but no new cells are formed.
- **Implications for Growth**: The growth of the organism is thus not due to an increase in cell number but rather an increase in the size of existing cells.
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