DESCRIPTION OF SPECIES 59 



preserved colonies are generally pale grey or almost white, with a pattern of darker spots marking the 

 positions of the zooids. A few slit-like common cloacal openings are present on the surface. All parts 

 of the common test are crowded with spicules, which are present even in the most superficial layer, 

 there being no bladder cells such as are often found in the uppermost layer of the test in didemnids. 

 The spicules are mostly of a regular stellate form, reaching 40 fi in diameter and having from eight to 

 twelve pointed conical rays in optical section (Text-fig. 12B). A few spicules were also seen consisting 

 of bundles of very thin needle-like rays arranged in a stellate pattern, these spicules also being 30- 

 40 /i in diameter (Text-fig. 12 C). A system of common cloacal canals lies a little above the middle level 

 of the colony. 



B 



3 0^ 



Text-fig. 12. Didemnum trivolutum sp.n. (St. WS 811): A, zooid; B, C, spicules. 



Zoom (Text-fig. 12 A). The thorax is about i-6 mm. long and the abdomen about i-o mm. The 

 prominent oral siphon has six pointed lobes. A large part of the dorsal side of the thorax is exposed by 

 the atrial opening which extends at least half the length of the thorax and often extends almost the 

 whole length. On the anterior border the opening bears a short triangular languet («./.). There are 

 large oval lateral thoracic organs with associated masses of small spicules. These organs occupy most 

 of each side of the thorax. 



Branchial sac. There are about eleven long narrow stigmata in each of the four rows. The three 

 dorsal languets are long, curved, and directed anteriorly. 



8-2 



