CILIARY MECHANISMS ON THE GILL IN AMPHIOXUS. 37 



then this mucus must be passed straight into the dorsal groove. The 

 cilia on the dorsal tubercle of Ciona when examined under a microscope 

 appear to lash away from the body-wall towards the lumen of the 

 branchial opening. Thus in the whole animal it is not improbable 

 that thin sheets of mucus are constantly passing from the dorsal 

 tubercle into the dorsal groove. These sheets would serve to entrap 

 food-particles in the same way as the ciliated tracts do in Amphioxus. 

 It is hoped, however, to complete these observations on a suitable 

 animal and to obtain more definite information on this subject. 



THE MAINTENANCE OF THE PHARYNGEAL SPACES 

 IN ASCIDIANS AND AMPHIOXUS. 



The maintenance of a current of water through the body of these 

 animals is dependent upon the maintenance of a continuous open passage 

 through the animal as well as upon the action of the gill as a water pump. 

 In Ascidians doubtless the main function of the test is to maintain a cavity 

 for the branchial sac. The body wall in Ascidians is maintained in close 

 contact with the test by organic connexions, so that the branchial sac lies 

 free in a cavity — the peri-branchial cavity — which the expanded pharynx 

 does not quite fill. Thus a cavity in communication with the pharynx 

 and the exterior is maintained outside the pharynx. The pharynx in 

 Ascidians is kept open by the intercrossing of longitudinal and transverse 

 bars of a sufficiently rigid consistency. In Amphioxus the pharyngeal 

 bars are supported, as is well known, by definite skeletal rods. The 

 peri-branchial cavity in Amphioxus, however, is maintained by dorso- 

 lateral sheaths of connective tissue arising from the notochordal sheath 

 (see Lankester, 16, Plate 36) held in position above the pharynx and 

 connected to the lateral surface of the notochord and dorsal skeletal 

 system by the myotomes. A glance at a pharyngeal section of this animal; 

 indicates at once how the attachment of the myotomes to the dorsal 

 region of the body and the dorso-lateral sheaths in the wall of the peri- 

 branchial cavity maintains this cavity. Indeed, the shape of the 

 myotomes in Amphioxus may have been directly influenced by their 

 function of assisting in maintaining a cavity in the ventral region of 

 the body. 



THE CILIATION OF THE GILL OF BALANOGLOSSUS. 



Up to the present it has been possible to examine only sections of the 

 gill of this animal, but it is hoped that observations may be made on the 

 living animal later. Examination of sections of the gill-bars of Balano-- 



