A COMPARATIVE ULTRASTRUCTURAL ANALYSIS OF SPERMATOGENESIS IN NEMERTEAN WORMS

Spermatogenesis has been analyzed by electron microscopy in eleven spe cies of nemertean worms. Nemertean testes are serially repeated sacs t hat contain germ cells as well as somatic cells of enigmatic function. In members of the class Enopla, intragonadal muscle cells and distinc t clones of developing sperm are typically present in the testes, wher eas such muscles and clones tend to be absent or less conspicuous in s pecies belonging to the class Anopla. During spermiogenesis, either a compact (< 5 mu m long) or an elongate (6-40 mu m long) sperm head dev elops, and an acrosome forms at the anterior end of the head. The matu re spermatozoon of all benthic species examined also possesses a mitoc hondrial component and a tail consisting of a single flagellum with a 9 + 2 arrangement of microtubules. In the pelagic enoplan Nectonemerte s, however, numerous flagella occur in the testes but are rarely obser ved attached to sperm heads, owing either to poor preservation or the possible sloughing of tails during spermatogenesis. The morphologies o f the sperm produced by various species seem to be related to the mode s of fertilization, as compact-headed sperm are associated with extern al fertilization, and elongate-headed sperm are often found in species that utilize internal fertilization or pseudocopulation. However, som e nemerteans utilize an external mode of fertilization and yet produce elongate-headed sperm. The possible significance of such sperm is dis cussed.