Owenia collaris embryos undergo holoblastic spiral cleavage to form a
coeloblastula, which, although exceptionally hollow (again like some nemerteans), conforms to the ancestral pattern for annelids (Okada, 1970).
A
coeloblastula is usually formed with the blastomeres comprising a single layered epithelium enclosing a large blastocoele (Nyholm, 1949; Mergner, 1971; Campbell, 1974; Spaulding, 1974; Fautin et al., 1989).
The blastomcres became ever smaller due to repeated cleavage, and after 8 h, a coeloblastula consisting of many cells and one cell layer was formed (Figs.
The zygote then undergoes radial, holoblastic cleavage and forms a hollow coeloblastula. Gastrulation follows by invagination to form a ciliated, pelagic planula larva.
Although the
coeloblastula is a major type of sponge larva, our knowledge about its development is scanty.
The even, asynchronous cleavage results in a
coeloblastula, which transforms into a disphaerula larva (Ereskovsky and Gonobobleva, 1999), with a diameter between 120 and 150 [micro]m (Fig.
In Demospongiae, several authors have suggested that the
coeloblastula or parenchymella larvae express an existing polarity in their attachment to the substrate.