lO 



up the mib-rib. These are especially numerous in the distal part of the peristome; and the 

 constant elongation of the mid-rib produced in this way thus results in the acquirement of 

 more and more obliquity in the disposition of the tube-lamellae as growth proceeds. 



The structures vvhich have so far been described may be distinguished as the "primary 

 lamellae". Cephalodiscus differs from Rhabdopleura in laying down additional series of "secondary 

 lamellae', both on the inside and on the outside of the primary lamellae. On the inner side, 

 these have a longitudinal arrangement (fig. 13), and their formation can be accounted for by 

 the supposition that the zooid lines its tube by additional lamellae secreted by its proboscis. 

 The external secondary lamellae may appear as a series of longitudinal layers running down 

 the outside of the peristome (figs. 12, 13), or as a series of layers bridging over the interval 

 between two diverging peristomes. Fig. 13 shews two young peristomes, of which that on the 

 left is viewed laterally, while that on the right is seen from the abaxial side. Examining the 

 angle between the two peristomes, it will be noticed that growth took place for some distance 

 without the formation of any bridging lamellae; and this arrangement can be made out in most 

 cases. After a time, however, intermediate lamellae are deposited; and some of these appear 

 to be derived from each of the two neighbouring peristomes. On the extreme left side of the 

 figure, some of the external secondary lamellae are seen as prolongations of the primary lamellae; 

 but many of them are certainly independent formations. It is probable that the extraordinary 

 length of the neck-region of the zooid, and the great mobility of the proboscis, in this species, 

 are correlated with the length of the peristomes, the animal having the power of stretching its 

 proboscis a certain distance down the outer side of its tube, during the deposition of these 

 external lamellae. 



The laminated structure of the coenoecium is further illustrated by figs. 108 — i 10 (PI. IX), 

 representing sections transverse to the principal axis of the entire colony. Fig. 109 shews parts 

 of seven zooecia ^), most of which are cut more or less transversely ; although c and e are cut 

 longitudinally, c through the wall of its peristome. In fig. 1 10, which shews the details of part 

 of the same section, b points to one of the primary lamellae of the zooecium ^; and it will 

 be noticed that the primary lamellae are covered both internally and externally by secondary 

 lamellae, some of the external ones being clearly contributed by the jjeristome c. The primary 

 lamellae (/r. /.) of the zooecium d are also distinguishable. The zooecia d and f are separated 

 from one another by a single series of primary lamellae (//'. /'.), indicating that the zooids do 

 not each secrete a complete wall of their own, but economise material by making what use 

 they can of the tubes of their neighbours in constructing their own zooecia. 



Fig. 108 is a section of the basal region of another peristome, with an e.xtensive 

 development of both internal and external secondary lamellae. The two primarj' lamellae marked 

 /;'. /. are cut along nearly their whole length ; and their sutural junction on the axial side is 

 indicated in the lower part of the figure. 



Foreign inclusions are not very numerous in the coenoecium of this species, but they 

 are by no means absent, as is indicated by figs. 12, 13. 



l) 1 use the term "zooecium" without iniplying any homology with the structure known by tlie same name in Polyzoa. Tlie 

 term is etymologically applicable to Cephalodiscus^ while its use in the Polyzoa may be defended merely on groimds of convenience. 



