51 



al\va)s found the stalk directed a\va\- from the mouth, except in cases where the metasome or 

 the stalk itself is bent towards the mouth (cf Pi. IV, fig. 40). 



The structure of the stalk of C dodecalophus has been described by M'Intosh (87, p. 20, 

 PL IV, fig. 5, etc.) and by Masterman (98, 2, p. 513, Pi. I, fig. 18). Masterman adds to the 

 general description given by M'Intosh the account of three definite nerve-tracts on the "ventral" 

 side of the stalk, and of a "dorsal" and "ventral" blood-vessel running along- the remains of 

 the mesentery found on opposite sides of the stalk. Each of the nerve-tracts is said to have 

 a fine lumen. 



I can confirm the accuracy of Masterman's account, e.xcept that I have not found a 

 lumen in any of the nerve-tracts, and that I prefer to use the terms "anterior" and "posterior" 

 instead of "ventral" and "dorsal" respectively. 



The stalks in the "Challenger" material are very strongly contracted, as is indicated 

 by the folding of the epidermis and of its basement-membrane as seen in longitudinal sections 

 (PI. XIII, fig. 169). These folds are responsible for the irregularities in the outline of the 

 transverse sections. PI. XI, fig. 133 shews the three nerve-tracts («./.) described by Masterman. 

 These appear to be continuous with a general nerve-ple.xus lying at the base of the entire 

 ectoderm of the stalk. The basement-membrane, internal to the nerve-layer, is deeply stained 

 with haematoxylin, although its thickness appears greater than it really is in consequence of 

 the e.xtensive folding which has been alluded to. The membrane is prolonged inwards in the 

 two median lines to support the anterior limb [a. v.) and the posterior limb {/>. v.) of the loop of 

 the ventral vessel which descends into the stalk. These prolongations of the membrane represent 

 remains of the ventral mesentery, which is complete in the young buds (cf. Masterman, 98, 2, 

 p. 515), but cannot be recognized as a complete partition in the adult stalk. The remainder 

 of the body-wall is constituted by a powerful development of longitudinal muscles, and the 

 interval left free from muscles is entirely filled by an extensive development of connective tissue. 

 The body-cavity of the stalk is thus virtual in the adult zooid. 



The stalk of C. levinseni (PI. XI, fig. 132) differs in certain respects from that of C. 

 dodecaloplucs. It is more regular in outline, and its nerve-layer forms a single projection, triangular 

 in transverse section, towards the interior of the stalk. The layer of longitudinal muscles is 

 arranged very definitely, as a pair of oval groups of fibres, open towards the middle line. 1 he 

 connective tissue filling the cavity of the stalk is not so much developed as in C. dodecalophus. 

 The two parts of the vascular loop are present as in that species, but the basement-membrane 

 is not so thick. 



The mode of origin') of the stalk from the body is explained by figs. 125 — 128. In the 

 sections nearer the mouth (PI. X, fig. 125), the muscular layer is relatively thin, and surrounds 

 the anterior third of the section, being divided into two halves by the ventral mesentery. In 

 PI. XI, fig. 126, the ventral mesentery has become incomplete, ending in a connective tissue 

 layer which separates the internal tissue of the .stalk from the general body-cavity. A small 

 triangular space, doubtless part of the body-cavity, intervenes between the part of the mesentery 



I) The other species agree, in essential respects, with C. levinseni.^ so far as the mode of origin of the stalk is concerned. 



