198 DISCOVERY REPORTS 



lates in the external layer and a few refractive beads also appear in the dorsal wall, i.e. 

 the side opposite the ventral thickening. 



The progressive diminution in the quantity of yolk causes the outer surface of the yolk 

 mass to shrink from the external layer, thus leaving a considerable space between the 

 two layers. At this stage (Harmer, 1905, pi. xiv, figs. 198, 199, 208), the yolk mass is at- 

 tached to the outer layer only by a short stalk-like process at one end, which is probably 

 the posterior ; so that absorption takes place only through the cells in this region. The 

 real coelomic cavities originate only at a later stage in development: the five body 

 cavities of the embryo described by Gilchrist (19 17) seem only to be parts of this space, 

 formed between the yolk mass and the external layer, which have been mistaken for 

 true coelomic pouches. 



LARVAL STAGES 



When the greater part of the yolk is absorbed, the blastocoelic cavity becomes an 

 empty space traversed only by a few protoplasmic strands, and the larva shrinks con- 

 siderably in size. An invagination now commences in the region where the stalk of the 

 yolk mass was originally attached. By this process a typical gastrula is formed. The 

 gastrula stage has been recorded by Andersson ; but its subsequent development, the 

 formation of the mouth and the transformation of the ventral thickening into the pro- 

 boscis region, is at present unknown. The youngest larvae which I have obtained re- 

 present a rather advanced stage in which the mouth is already formed and the proboscis 

 region is well marked (Plate XLIII, fig. 2). The outer wall of the proboscis is very thick 

 and is formed of large glandular cells, which stain very deeply in haematoxylin and in 

 which masses of brown pigment are very conspicuous. The collar region on one side is 

 very prominent and the ectoderm of the trunk and collar is formed of large columnar 

 cells, which carry numerous refractive beads. These are homogeneous transparent bodies 

 which are embedded in the cells of the ectoderm. In the adult zooid of C. hodgsoni and 

 C. dodecalophiis they are found at the tip of the arms, while in C. densus they are found in 

 the dorsal wall of the proboscis and in the dorsal wall of the arms. Both in the adult 

 zooids and in the larvae they are sometimes found partially extruded from the surface of 

 the ectoderm. Throughout the larval stages and in the young zooids which develop from 

 the larvae the refractive beads are found in large numbers in the ectoderm of the trunk ; 

 but when the young zooid settles down and begins to construct the coenoecium, they 

 disappear in the body wall and become restricted to the tips of the arms. It is therefore 

 possible that these refractive beads are the main defensive organs during the free- 

 swimming larval stages of development. The fact that they are not found on the trunk 

 or arms of the young buds constitutes one of the chief differences between the habits and 

 structure of the two. 



The gut opens externally through a narrow mouth situated antero-ventrally, and 

 its walls are formed of cells which are somewhat shorter than those of the ecto- 

 derm. The mouth is roofed over by the inner wall of the proboscis, and its floor is 

 formed by a short swelling of the anterior end of the trunk, which represents the 

 future post-oral lamella. The mouth leads into a wide cavity, which becomes slightly 



