This is a picture of the 5-day old planula larva of P. flavicirrata. As in other hydrozoans and scyphozoans, the planulae of P. flavicirrata are lecithotrophic - they are non-feeding, rather they depend on yolk reserves to reach metamorphosis. Planulae are ovoid in shape and uniformly ciliated. Many hydrozoan planulae develop muscles only after metamorphosis (S. Maslakova, pers. com.), but in my culture, planulae of P. flavicirrata were clearly contractile!
Believe it or not, this is the same individual as shown above. It has used its muscles to elongate. Hydrozoan planulae have two types of epithelial muscle cells that are separated by mesoglea (Gröger 2001).
A close up view of the same individual shows the two cell layers: the outer ectodermis and the inner endodermis, separated by a thin extracellular layer of mesoglea. Myoepithelial cells in ectodermis and endodermis have muscle fibers (not detectable without special staining) that run along the mesoglea.
Gröger, H and V. Schmidt. 2001. Larval Development in Cnidaria: A Connection to Bilateria? gensis 29 (3):110-114.
Mills, C.E. and J.T. Rees. 2007. Key to the Hydromedusae. In "The Light and Smith Manual Intertidal Invertebrates from Central California to Oregon". Edited by J.T. Carlton. University of California press. Los Angeles. Pp. 137-150.
Wrobel, David and Claudia Mills. 1998. Pacific Coast Pelagic Invertebrates A Guide to the Common Gelatinous Animals. Monterey Bay Aquarium and Sea Challengers: California.