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 which have so far been described may be distinguished as the "jjrimary 

 lamellae". Cephalodisciis differs from Rhahdopletira 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. 1 3 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 depo.sited; 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 tulie, during the deposition of these 

 e.xternal 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 '(a'-^. \ 10, which .shews the details of jjart 

 of the same section, b points to one of the jn-imary lamellae of the zooecium c\ and it will 

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

 lamellae, some of the e.xternal ones being clearlv contributed b\ the jjeristome c. The primary 

 lamellae {pr. I.) of the zooecium d are also distinguishable. The zooecia d and / are separated 

 from one another by a single series of jjrimary lamellae {pr. /'.), indicating that the zooids do 

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

 thev 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 extensive 

 development of both internal and external .secondary lamellae. The two primary 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. 



I'oreign inclusions are not ver\' numerous in the coenoecium of this species, but they 

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



1) I use the term "zooecium"' without implying any homology with the structure known by the same name in Polyzoa. The 

 term is ctymologically applicable to Cephaloiiisciis^ while its use in the Polyzoa may be defended merely on grounds of convenience. 



