DESCRIPTION OF SPECIES 85 



Occurrence. St. 123: S. Georgia, 230-250111. St. 144: S. Georgia, 155-178111. St. 160: Shag 

 Rocks, 177 m. St. 371 : S. Sandwich Islands, 99-161 m. St. WS 27: S. Georgia, 104 m. St. WS 243 : 

 Patagonian Shelf, 144-141 m. St. WS 841 : Patagonian Shelf, 110-121 m. 



Colony. The largest specimen in the collection is one from the Patagonian Shelf (St. WS 841), 

 measuring 9 cm. tall, 1-5 cm. thick, and 7 cm. long. The original colony must have been still larger, 

 as the specimen is only a portion. In form the colonies (PI. Ill, fig. 1) are generally pillar-like, wedge- 

 shaped, or club-shaped and are often markedly flattened from side to side. The test is firm but not 

 hard, sometimes almost clear and occasionally nearly opaque. Some of the colonies from South 

 Georgia and the Patagonian Shelf are reddish brown in colour. In most specimens the zooids are 



A _ 



d.p. H 



0.2 mm 



2.0 mm 



Text-fig. 25. Tylobranchion speciosum Herdman: A, zooid (St. WS 27); B, C, D, branchial papillae from different specimens 

 (Sts. WS 27, 371, WS 243); E, gonads; F, transverse section of zooid at level of heart ; and G, below level of heart; H, larva 

 (St. 144). 



readily visible through the test. Attachment of the colony is usually to a small group of pebbles 

 embedded in the basal test. 



Zoom (Text-fig. 25 A). The general structure of the zooid is already known, and the principal 

 variations in the ' Discovery ' specimens are tabulated on page 86. Certain points regarding the lower 

 part of the zooid, however, have remained doubtful. Berrill (1935) described a true post-abdomen 

 containing the heart and gonads, but Arnback (1927, 1950) showed that the heart is much farther 

 forward than Berrill's figure indicated. A careful examination of the ' Discovery ' specimens allows 

 me not only to confirm Arnback's findings, but also to add details concerning the epicardium and the 

 nature of the posterior part of the zooid. The epicardium is a flattened tube forming a partition across 

 the abdomen. Extending from just below the base of the thorax, it reaches the pericardium (pc.) but 

 instead of ending there it continues into the lower, narrow part of the zooid where it forms a partition 

 separating the two blood channels (Text-fig. 25 G, b.c). This part of the zooid behind the heart also 



