102 THE GASTROPODA 
Nucula, and is similarly free to a greater or less extent at its 
distal extremity. But in Scisswrella (Fig. 54, IV) the right etenidium 
is already degenerate in so far that it has only a single row of 
filaments inserted directly on the wali 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 
Fic. 82. 
Anterior part of the body of Acmaea, showing nervous and circulatory systems, dorsal 
aspect. «.g, abdominal ganglion; ao, aorta; au, auricle; b7.n, branchial (ctenidial) nerve ; 
br.v, branchial vein; ce.g, cerebral ganglion; gi, gill; 7.7.9, infra-intestinal ganglion; mu, 
columellar muscle ; 0s’, 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 ; 
we, 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 without any internal 
endothelial 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 
