SYSTEMATIC ACCOUNT 243 



any of the three sub-genera previously recognized. The absence of a definite cavity 

 forming tubular spaces individually or occupied in common by the zooids is unique. 

 The spaces between the bars and spines on the surface of the meshwork are the openings 

 through which the zooids come out and are not homologous with the ostia of other 

 species, being only temporary outlets, which become part of the internal meshwork 

 when new bars are formed above the existing ones. 



Most of the colonies are attached to calcareous tubes of Polyzoa^. Some colonies are 

 attached to the tips of these tubes, while in others they are horizontally embedded in 

 the basal region, surrounded by a thin layer of coenoecial substance. 



The se.xual development of Cephalodiscus has not been thoroughly worked out. 

 Harmer (1905, p. 102) described the embryonic stages of C. gracilis and C. levinseni, 

 and Gilchrist (1917) gave an account of the larvae of C. gilchristi, but nothing definite 

 is known about the metamorphosis and habits of the later stages of the larvae, as they 

 were not studied in the living condition. In spite of this ignorance, we may assume that 

 at the end of the free-swimming stage the larvae of C. kempi settle down on some suitably 

 firm support, such as the calcareous tubes of Polyzoa, and secrete a thin layer of coenoecial 

 substance which envelops part or whole of the supporting object. 



Rudiments of spines are laid down on the surface of this envelope which project at 

 various angles. As the spines increase in length, lateral bars are formed, which grow at 

 right angles to the axis and fuse with adjacent spines. A complicated meshwork is thus 

 produced by the formation of four or five cross-bars between each adjacent pair. By 

 the additional deposition of coenoecial substance the lateral bars and spines increase in 

 thickness, the angles become smooth and curved so that the original rectangular spaces 

 become circular or ovoid. As the zooids increase in number and more space is required, 

 new lateral bars are formed above, but the colonies do not grow beyond the tuft-like 

 form. 



Two growing colonies have been found attached to flat surfaces (one to the mantle 

 of an Ascidian, and the other to a broken piece of Lamellibranch shell). In both of these, 

 a flat, plate-like secretion was first formed. Colonies were also attached to fragments 

 of sponges. Whatever the object to which the colony is attached, a layer of coenoecium 

 is spread on the surface of the object, which as growth proceeds becomes partly or 

 wholly enclosed. 



Form B. This form is represented by material obtained from stations WS 71, WS 75, 

 WS 84, WS 88, WS 97, WS 211 and WS 216. Stations WS 97 and WS 211 gave most 

 of the collection, while from each of the other stations only one or two pieces were 

 obtained. 



Colonies are formed of massive cylindrical branching stems with a close array of 

 spines along the sides. The length, thickness and nature of the spines depend on their 



^ " The Polyzoon which is associated with C. kempi is Ogivalia elegans, the Vincularia elegans of D'Orbigny, 

 a species which is abundant in the ' Discovery' collections. This is a branching form with brittle branches. 

 It seems not unlikely that the Cephalodiscus was growing on it and that the dredge merely broke off the tips 

 of the branches." Letter from Sir Sidney Harmer. 



