72 OKHAMANDAL MARINE ZOOLOGY REPORT 



mesenteries is to be correlated with the little depth characteristic of the gill lamellse in 

 Placuna. 



The histological structure of the filaments is essentially the same as in Anomia 

 and other forms where the branchiae exhibit little specialisation. Each filament consists 

 of a single layered cylinder of cells supported on a delicate basal membrane. In section 

 the cylinder is seen to be somewhat club-shaped (fig. 34) ; the broader end morphologi- 

 cally is the ventral side, the narrower the dorsal. The cells of the former portion are 

 very large, those of the narrow end exceedingly small and unciliated. 



The majority of the cells on the broader portion of each filament are ciliated ; 

 certain stable distinctions in the distribution and character of this ciliation divide 

 the ventral or exterior part of the filament into longitudinal tracts. A transverse 

 section of a filament shows these to consist of — {a), a wide frontal ciliated tract 

 comprising all cells facing directly outwards ; the cilia on these cells are particularly 

 short {F-c.) ; (6), at the antero-lateral corner on either side of the frontal tract, 

 a single large cell bearing many much elongated cilia {A.l.c), and containing a 

 specially large nucleus ; (c), a narrow bare region on either side of the filament, 

 bounded outwardly by the ciliated " corner-cell," and on tlie inner aspect by the 

 lateral cilia mentioned next; {d), a lateral ciliated tract {L.c.) several cells wide 

 bearing long cilia, and (e), a few cells between the lateral ciliated tract and the 

 margin of the small cells of the dorsal (internal) section of the filament. Fig. 34 

 makes clear this disposition of the ciliated tracts. 



Beneath the epithelium of the filament a layer of connective tissue lines the 

 cavity ; for the most part this layer is thin, but within the narrow dorsal section 

 of the filament it thickens and strengthens into two stout longitudinal skeletal 

 bands (Sk.h.) to constitute a supporting framework against the collapse of the 

 filamental tube. 



As in Anomia, the cavity of the filament is divided into two somewhat unequal 

 channels by an extremely delicate septum (Sep.) uniting the two skeletal thickenings 

 towards the middle of the filament and situated rather nearer the narrower than 

 the broader face. 



The smaller or dorsal of these small vessels serves as an afi'erent (A.ch.), the 

 larger or ventral as an eff'erent blood channel [E.ch.) through the filament. 



At the distal end of each reflected filament these two channels communicate, 

 the afi'erent passing over into the eff'erent, as the dividing septum does not extend 

 quite to the extreme end of the filamental cavity. 



Bkanchial Ciectjlation. — As a consequence of the simplicity of the branchial 

 plan, the blood circulation is for the most part of an equally primitive type. 

 Impure blood from the body generally, after having bathed the tubules and pouches 

 of the renal body, is collected into two well-defined vessels, the afi'erent branchial 

 sinuses (figs. 4 and 15, C.hr.aff.), one of which lies within each mesentery, entering 



