5 8 DISCOVERY REPORTS 



Branchial sac. Ten to twelve long stigmata are present in each of the four rows. The stigmata are 

 generally of the width shown in Text-fig. 1 1 , but in a few exceptionally well expanded zooids they are 

 much wider and rectangular in shape. The three dorsal languets are long, and usually forwardly 

 directed. 



Gut. The oesophagus is narrow and about as long as the stomach, which is a wide barrel-shaped 

 organ. The post-stomach is about equal in length to the intestine, and the wide rectum ends in a two- 

 lipped anus situated at the base of the thorax, opposite the 4th row of stigmata. 



Gonads. In most zooids the ovary is not developed, and when present it is represented by one large 

 ovum, and a group of smaller ova. The testis consists of two or three rounded follicles (t.) on the 

 surface of which the sperm-duct makes four to six spiral turns. 



Developing embryos were present in the common test below several of the zooids, but none had 

 reached the larval stage. 



Remarks. I have had some difficulty in deciding the identity of these specimens, mainly owing to 

 the complete absence of spicules. This feature may be an artifact, as suggested above, or may repre- 

 sent the extreme reduction of spicules which previous authors (van Name, 1945; Kott, 1954) have 

 noted as being scarce in some colonies which they examined. It is relevant that considerable variation 

 is known in the abundance of spicules in other species of Didemnidae (e.g. Trididetnnum tenerum 

 (Verrill), van Name, 1945, Berrill, 1950; T. opacum (Ritter), van Name, 1945 ; Didemnum helgolandicum 

 Michaelsen, unpublished personal observation). 



A comparison of the 'Discovery' specimens with the descriptions given by Sluiter (1906), Hart- 

 meyer (191 1), van Name (1945) and Kott (1954) is given in Table 16. 



The differences shown above seem less important than the similarities, especially considering the 

 doubtful nature of the absence of spicules, and I conclude that the colonies are within the probable 

 range of variation of D. biglans. 



Distribution. Antarctic (Graham Land, Enderby Land, Kaiser Wilhelm II Land). 



Didemnum trivolutum sp.n. (Text-fig. 12) 



Holotype. St. WS811. 



Diagnosis of species. Colony thin and encrusting, with spicules densely packed throughout the 

 whole depth of the test. Spicules of a regular stellate shape, 20 to 40 /i in diameter with eight to 

 twelve rays in optical section; also some spicules with very thin needle-like rays. Zooid with wide, 

 long atrial opening, surmounted by a languet; lateral thoracic organs large; stomach rather rect- 

 angular; testis undivided, with sperm-duct making three spiral turns. 



Occurrence. St. WS 99: Falkland Islands, 251-225 m. St. WS 811 : Patagonian Shelf, 99 m. 



Colony. The specimens consist of large sheeting masses, sometimes growing round the tubes of 

 worms, and reaching a length of 10 cm. The average thickness of the colony is 3 mm. In colour the 





