SYSTEMATIC ACCOUNT 235 



zooid of C. nigrescens never completely leaves the tube. It moves forward towards the 

 ostium and may even push its body out, but the stolon always remains fixed to some 

 point on the inside of the tube, so that on the slightest danger the body is withdrawn 

 into the tube (of. " Observations on living zooids," Andersson, 1907). This habit makes 

 the elongation of the stalk essential for the proper freeing of the buds. The buds when 

 first formed are sessile. They develop slender stalks which rapidly elongate upwards, 

 the buds remaining as thin flat plates. The lengthening of the stalks pushes the buds 

 through the narrow space between the body of the mother and the wall of the tube, and, 

 reaching beyond the arm-tips of the mother, the buds break away as free individuals. 



Serial sections were cut of selected zooids from representative pieces of colonies, but 

 no new observation that is worth recording has resulted. The blackish colour of the body 

 is caused by a superficial layer of large brown cells. They occur in great abundance also 

 on the dorsal wall of the proboscis. 



The proboscis, which is attached by its dorsal part to the collar region, is oval with 

 indented sides. It measures i to 1-2 mm. along the antero-posterior axis, and about 

 0-8 to 0-9 mm. from side to side. The curved red line runs horizontally between the 

 indentations. The region just above and below the red line is lighter, the pigmentation 

 being deepest on the anterior and posterior edges. The middle part of the ventral face 

 of the proboscis is thick and stains deeper than the rest of the body. In longitudinal 

 sections passing through the middle of the proboscis one or two narrow invaginations 

 are found. These may be tracks through which the coenoecial substance formed by the 

 glandular proboscis flows out. 



There are usually seven pairs of arms, but zooids with six pairs are also found. They 

 are devoid of end-swellings with refractive beads and possess two dark longitudinal 

 bands along the axis. Towards the tips the two bands approach one another and fuse 

 into one broad band. 



In general, the internal structure of the zooid of C. nigrescens agrees with that of all 

 other species. For systematic purposes the only noteworthy diff'erence is the position 

 of the heart and pericardium relative to the notochord. In C dodecalophus the heart 

 and pericardium are placed at the tip of the notochord, but in C. «/]§'i'e5ce«s the pericardial 

 sac is attached to the notochord subterminally. 



Sub-genus Orthoecus 



Cephalodiscus densus, Andersson (PI. XXXV, fig. i). 



Andersson, 1907. 



Ridewood, 1918 b. 



C. ranis (Andersson), Ridewood, 1918 a. 



Diagnosis. Colony not branching, a bulky mass or cake, composed of closely set, 

 vertically directed tubes of uniform diameter (i to 1-2 m.), measuring up to 70 mm. in 

 height. Colourless or pale brown. Longest tubes (which are found towards the centre 

 of the colony) 60 to 70 mm. ; those towards the edges, shorter. Ostia transverse and 



