|Placenta of a salp (Salpa fusiformis)|
Reproduced from Bone, Pulsford and Amoroso (here)
(C) 1985 with permission from Elsevier
The life cycle involves alternation between asexual and sexual generations. The asexual phase (oozooid) develops within the jet chamber of the sexual phase (blastozooid). The image above is from Amoroso's last paper, published almost three years after his death (see previous post for Amo). The placenta consists of two layers: an outer cortex (co) and an inner central layer (c). These separate the embryonic (E) and maternal (M). circulations. Both layers are syncytial and both are maternal in origin. However, embryonic leucocytes pass into and add to the cortical layer.
Blastozooids are hermaphrodite, but the egg develops before the testis matures so is fertilized by sperm from a different blastozooid
|HMS Rattlesnake on which Thomas Henry Huxley served as |
Assistant Surgeon during the voyage to Australia and New Guinea 1846-50
National Maritime Museum (public domain)
We tend to think of placentation in terms of mammals, reptiles and fish, but a current paper in Biological Reviews (here) shows that maternal provision of nutrients (matrotrophy) and even placentation is not infrequent in invertebrate phyla.