Spermatocytes develop from the transformation of choanocytes and oocytes arise from archeocytes. The mesenchyme lines the region between the pinacocytes and the choanocytes. The inner part is formed by cells called choanocytes, equipped with a flagellum. The cell body of choanocytes is what is responsible for nutrient absorption. However, a few carnivorous sponges have lost these water flow systems and the choanocytes. Choanocytes bear a superficial resemblance to Choanoflagellates. Most known carnivorous sponges have completely lost the water flow system and choanocytes.