112 PHYLUM ENTEROPNEUSTA. 



The common axis consists of the stalks of the zooids in earlier 

 stages of the growth of the colony. It lies within the tube and 

 the older parts of it possess a cuticle on its ectoderm. This 

 cuticle becomes hardened and of a dark colour. It fuses second- 

 arily with one side of the wall of the tube and forms a con- 

 spicuous object as a dark brown, thin rod easily visible to the 

 naked eye in the creeping stolon of the colonies. Some of the 

 terminal branches of the colony do not rise up, but the tubes 

 with their contained axis continue along the substratum and end 

 in open mouths. These are actively growing proliferous branches. 

 The axis in them terminates in an imperfect zooid, the proboscis of 

 which adds to the tube, while its stalk has lateral wart-like buds. 

 Transverse septa are formed across the tube between the buds, 

 and the buds increasing in size burst through the wall of the tube 

 and grow outwards from the creeping axis as imperfect zooids. 

 Most of the new zooids so formed rise up and develop into the 

 perfect form ; a few no doubt adhere to the substratum and 

 form new proliferous branches. It follows from what has been 

 said that the creeping (stolonic) part of the tube is divided up 

 into chambers by septa which do not however interrupt the con- 

 tinuity of the living substance of the stalk, and from each of 

 which one zooid-bearing branch arises. There are no branchial 

 apertures, but there is on each side a groove, between the inser- 

 tion of the proboscis and that of the arms, which leads through 

 the mouth into the oesophagus, on the side-wall of which it can 

 be traced. This groove is called the branchial groove. 



The anatomy of the animal may be understood at a glance by 

 inspection of Fig. 82. The alimentary canal is bent on itself. 

 The buccal cavity is provided with an anteriorly directed diverti- 

 culum which is continuous with a rod-like structure, apparently 

 half-cellular and half-gelatinoid (Fig. 82, 9). This, which is 

 clearly comparable to the notochordal diverticulum of other 

 Enteropneusta, projects into the base of the preoral disc (pro- 

 boscis). The body cavity, which appears to have a cellular 

 lining, is divided into five chambers arranged as in other Entero- 

 pneusta ; viz. an unpaired chamber (10) in the preoral disc (pro- 

 boscis), a pair of chambers separated from one another by dorsal 

 and ventral median septa and placed in what may be called the 

 collar region of the body (Fig. 82, 2), and finally a pair of chambers 

 in the trunk in relation with the greater part of the alimentary 



