102 THE GASTROPODA 



Nucula, and is similarly free to a greater or less extent at its 

 distal extremity. But in Scissurella (Fig. 54, IV) the right ctenidium 

 is already degenerate in so far that it has only a single row of 

 filaments inserted directly on the wall of the pallial cavity. The 

 other ctenidiate Aspidobranchs have only a single and equally 

 bipectinate ctenidium, as has also Valvata (Fig. 132, gi). The two 

 rows of respiratory filaments are equal in the dibranchiate 

 Rhipidoglossa, in the Acmaeidae and Valvatidae, but in the mono- 

 branchiate Rhipidoglossa the dorsal row that is to say, the row 

 between the mantle and the branchial axis is already much 

 reduced, and in the remainder of the Streptoneura this row of 

 filaments has disappeared, as in the right ctenidium of Scissurella, 

 and the single ctenidium is attached to the mantle for the whole 

 of its length (Fig. 99, XVII). The individual branchial filaments are 



\n, 



Anterior part of the body of Acmaea, showing nervous ami circulatory systems, dorsal 

 aspect, a.g, abdominal ganglion ; ao, aorta ; era, auricle ; br.n, branchial (ctenidial) nerve ; 

 br.v, branchial vain ; ce.g, cerebral ganglion; gi, gill; i.i.g, infra-intestinal ganglion; mu, 

 columellar muscle ; os', os", left and right osphradia ; pa, mantle ; pa.v, pallial vein ; pe.c, pedal 

 cord ; pe.g, pedal ganglion ; pl.g, pleural ganglion ; s.i.g, supra-intestinal ganglion ; te, tentacle ; 

 ve, ventricle. 



usually simple, but sometimes their surfaces are folded, and again 

 each filament may be in its turn leafy or beset with plications as 

 in the Cephalopoda : this condition is found in Janthina. Each 

 filament is a simple tegumentary projection Avithout any internal 

 ndothelial lining. The wall of the blood -space contained in it 

 is formed of connective tissue, thickened and compacted along the 

 borders of the filament, where it forms a supporting structure, 

 specially well developed on the ventral side. By these means 

 the rigidity of the filaments, which are often very long as in 

 Calyptraea, etc., is ensured. The cavities of the filaments are 

 traversed by muscular trabeculae, by whose agency the whole 

 filament may be contracted. 



In spite of the presence of ctenidial branchiae, there is a certain 

 number of Gastropods in which the oxygenated blood returned to 

 the auricle is not derived from these organs alone. A considerable 

 quantity may come from various other parts of the mantle, or, in 



