47 



limited circulation of the blood through the filaments 

 themselves. 



There is a very regular segmental or repetitional 

 structure in each branchia, which is due to the fluting 

 and to the regular occurrence of the afferent and efferent 

 vessels. At the base of each furrow is a vessel, the 

 external wall of which is formed by the two flattened- 

 out filaments already referred to. The rest of the vessel 

 is formed from extra-filamentar tissue. Afferent and 

 efferent vessels so formed alternate with complete regu- 

 larity along the whole length of the branchia. There is 

 this difference between the two series : — the afferent series 

 (Br.aff' .) consists of a number of hollow plates extending 

 uninterruptedly from the base to the ventral edge of each 

 branchia, and also stretching across from external to 

 internal lamellae ; each afferent vessel is thus common to 

 the two lamellae of the branchia; the efferent series consists 

 of a number of nearly cylindrical vessels (Br.eff' '.), one of 

 which is present at the base of every alternate furrow. 

 They must necessarily be double the number of the 

 afferent vessels. The afferent vessels form the inter- 

 lamellar junctions. It also follows from this arrangement 

 that the suprabranchial cavity in the inter-lamellar space 

 is divided up into a series of separate cavities having no 

 connection with each other except at the base of the 

 branchia. 



Fig. 26 represents such a double segment in the internal 

 branchia of one ctenidium. There are from 50 to 60 such 

 segments in the length of the organ. 



The separate filament (fig. 27) is in transverse section 

 ellipsoidal in shape, the broader end is external, the more 

 pointed end is internal. The outer surface is composed of 

 large cubical cells, the inner surface of smaller cells. 

 Within is an elongated cavity across which bridges of 



