. 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
. 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, ). This part of the zooid behind the heart also
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