His restless ovipositor was eagerly pumping more into the shallow water behind him. Here he could finally admire them, until the waves came in and took them back home. And then...

Well, perhaps the oceans were overdue for another Cambrian Explosion.

His everted ovipositor was seeking a suitable place to deposit his brood. And so, after wrangling the writhing organ into a loose-fitting pair of trousers, its instinctive twitching became his guide.

The tide pools, several of them already overflowing with trilobites.

Without resistance, flesh billowed into his tunnel, filling him from the inside out, his heavy womb and its many occupants tumbling down, down, down…
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The bulge in his boxers throbbed and tented, straining against its fabric confines.

What began as another uterine twinge had intensified into a persistant squeeze. The weight in his pelvis had grown so heavy that it couldn't be lifted, no matter how much he tossed and turned, or how hard he kneaded the taut skin. The only hope now was to surrender his body to the next contraction.

Each trilobite moved on its own looping path, content to trace endless circuits around the tiny ocean of his womb; but every now and then, that rhythm would be broken by the shock of an antenna probing at his cervix, and the resultant clenching of his uterus would send its inhabitants scurrying.

Chitin glimmered, antenna waved, gills breathed - until at long last, the creatures swam once more!

His belly rolled slow and heavy with their movements, its shape subtly changing as they swam and crawled en mass.