Internal Features
Internal features of B. tuberatus is the main factor used to identify the ascidian down to the species level. B. tuberatus, like other ascidians, are fundamentally bilateral (Rocha & Kremer 2005). With respect to water flow, it first enters the incurrent/branchial aperture of the individual zooids as seen in figure 1 below. The branchial baskets of B. tuberatus are especially important in identification as it only consists of 4 rows of stigmata, with 3 stigmata in each mesh (compared with the 7-12 rows in other Botryllus species). However, it has been recorded that some individuals of this species may not adhere so closely to the number of rows stigmata, even though it would still be fewer than other Botryllus species (Kott 1985).
 Figure 1. Image showing a longitudinal section of a single B. tuberatus zooid. The blue arrow shows the direction of water flowing into the zooid through the branchial aperture and between the tentacles into the atrial cavity. The fragmented walls on the outside of the atrial cavity are remnants of the branchial basket.
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