Platynereis megalops may be found in Great Harbor and in Eel Pond at Woods Hole, Mass. The swarming heteronereis form is attracted to light and may conveniently be collected with the same type of flat net as is used to obtain Nereis limbata. The sexes are separate. The reddish males swim with rapid, jerky movements' rotating in spirals tangential to the surface of the water. The large females, pale yellow in color, swim slowly at a greater depth. They travel either in a straight line, or, with head bent at right angles to the body, describing a circle about the head (Just, 1914). The males are smaller than those of Nereis limbata (which swarm at the same times) and swim more rapidly. Although it may be somewhat difficult to distinguish these two species when swarming, they are easily differentiated when examined in the laboratory. The eyes of Platynereis are much larger and form conspicuous dark spots on the prostomium, which, unlike that of Nereis, lacks palps and protrudes anterior to the eyes as a transparent, oval lobe.

The animals rarely appear in large swarms, and as a rule, the number appearing during an evening can be easily counted.

July, and the first three weeks in August. Platynereis shows a lunar periodicity, appearing in varying numbers from full to new moon. The frequency curve does not correspond precisely to that of Nereis limbata. See the paper of Just (1914) for details of swarming.

A. Care of Adults: The animals should be isolated in separate fingerbowls of sea water as soon as they are collected. As with Nereis, trays of bowls containing clean sea water and a piece of Ulva should be used. The water should be changed in the laboratory, and the dishes placed on the sea water table.

B. Procuring Gametes: It is imperative to obtain gametes by allowing males and females to mate. Just (1914) succeeded in artificially inseminating eggs only when he mixed "dry" eggs with "dry" sperm. Artificial insemination was not successful, when the eggs were diluted with more than an equal volume of sea water. Mating, fertilization and subsequent egg-extrusion will occur when a male and female are placed together. The mating habits, as described by Just (1914), are of special interest; polyspermy does not ordinarily occur.

C. Preparation of Cultures: The animals should be allowed to mate as soon as possible after collection and the adults removed immediately following shedding. After 20 minutes, decant the water from the dish of eggs and replace with fresh sea water. At the time of the first cleavage, gently break up the jelly-mass and distribute it equally among 7 to 10 fingerbowls of fresh sea water. After about 8 hours, the water should be changed again, and when the trochophores become free-swimming, they should be transferred daily to fresh sea water. The trochophores of this form are markedly sensitive to light, and if too many are kept in one dish, clumping and consequent smothering will occur. One way to prevent this is to keep the larvae in subdued light.

NO feeding is necessary up to the three-somite stage, but as soon as all the endodermal oil drops are absorbed, diatom feeding should be initiated (Just, 1922).

A. The Unfertilized Ovum: The oöcyte from the body cavity is compressed and irregular. After they are shed, the few uninseminated eggs round up and become nearly spherical, although the polar axis remains slightly shorter than the diameter of the equator. The egg diameter varies somewhat, the largest measuring between 180 and 200 microns. The egg is almost perfectly transparent (much more so than the egg of Nereis limbata), with an equatorial ring of oil drops and a well marked, clear cortex containing very fine granules and striations.

B. Fertilization and Cleavage: Insemination follows the curious copulation phenomenon (see the paper by Just, 1914), and is internal; therefore, although the egg is shed in the germinal vesicle stage, it usually has a sperm attached, and a jelly layer and perivitelline space are forming at this time. NO sharply defined fertilization cone is present. Sperm penetration is completed about 30 minutes after egg-extrusion; the middle and tail piece are left outside the egg (Just, 1915b). Development is very similar to that of Nereis limbata.

C. Rate of Development: NO detailed information is available. Well-formed, swimming trochophores are present in 24 hours; by the seventh day the larvae have three setigerous segments bearing parapodia. After this time, at least one new segment is added daily.

D. Later Stages of Development: The larvae resemble those of Nereis limbata.

JUST, E. E., 1914. Breeding habits of the heteronereis form of Platynereis megalops at Woods Hole, Mass. Biol. Bull., 27: 201-212.

JUST, E. E., 1915a. An experimental analysis of fertilization in Platynereis megalops. Biol. Bull., 28: 93-114.

JUST, E. E., 1915b. The morphology of normal fertilization in Platynereis megalops. J. Morph. 26: 217-233.

JUST, E. E. 1922. On rearing sexually mature Platynereis megalops from eggs. Amer. Nat., 56: 471-478.

JUST, E. E., 1929. Effects of low temperature on fertilization and development in the egg of Platynereis megalops. Biol. Bull., 57: 439-442.

JUST, E. E., 1939. The Biology of the Cell Surface. P. Blakiston's Son & Co., Inc., Philadelphia.