.74 OKHAMANDAL MARINE ZOOLOGY REPORT 



frontal band of short cilia, bounded by (b) a narrow band of long lashing cilia at each 

 corner, (c) a lateral band of equally long cilia, and (d) close to the crest of the gill a 

 ciliated disc on each lateral face of the filament. 



The function of the last-named is to keep the filaments iii position, to prevent 

 them fraying out and becoming displaced. The short, stiff" cilia of the "discs" 

 interlock most securely and provide very serviceable unions between the filaments 

 (fig. 33). The cilia of the lateral bands are not localised to one section of the filaments 

 like the " discs," but stretch from end to end of the filaments ; they are very much 

 longer and have very much less intimate union — those from opposing faces appear to 

 lightly interdigitate only towards their extremities. Besides the support they provide 

 by this interlocking, slight and scarcely adequate though it appear, these cilia 

 function as the actual strainers of the food particles, and this may be considered their 

 particular duty. 



That of the lashing cilia of the corner cells is well known — it is to create and 

 maintain the incurrent water- stream. 



Finally, coming to the short cilia {F.c.) that clothe the whole frontal (ventral) 

 face of each filament, observation upon the living animal leads me to believe that the 

 special function here is the propulsion of food particles outwards to the branchial crest, 

 where, as in the pearl oyster, I have been able to recognise a ciliated pathway leading 

 directly to the base of the labial palps. 



Thus when a diatom or an algal spore is intercepted by the sieve formed by the 

 interdigitation of the long cilia of the lateral bands it is thrown out upon the broad 

 frontal path of short cilia which pick it up and propel it swiftly to the gill crest, where 

 the cilia of the crest pathway catch it and propel it at right angles to its first course 

 onwards to the anterior apex of the gills, where it is surrendered to the care of the 

 palps. 



THE VASCULAE SYSTEM. 



The vascular system of Placuna, in common with that of more typical Lamelli- 

 branchs, consists of a central organ of propulsion, the heart, of arteries having a definite 

 lining of epithelial cells, of irregular ill-defined spaces or lacunae, and of a well- 

 developed series of more or less well-defined and regularly disposed permanent channels 

 or sinuses, functioning in the main as venous trunks, but differing therefrom histo- 

 logically, as the walls of these sinuses are without an epithelial lining. The blood is 

 colourless. 



It is characteristic of the Anomiidse that the heart is not contained in a 

 pericardium, and this peculiarity is well seen in Placuna. Here the thick-walled 

 ventricle (F.) lies suspended freely in the mid-posterior region of thepallial cavity, just 

 above the posterior ventral angle of the visceral mass ; its relative position is dorsal to 



