THE AMPHINEURA 45 
coils. The anus opens at the posterior extremity of the body, 
between the mantle and the foot (Fig. 28, a). 
2. Circulation and Respiration.—The heart, enclosed in a large 
pericardium, occupies the postero-dorsal region of the body. It 
consists of a median elongated ventricle and two elongated 
symmetrical auricles. The openings of the auricles into the 
ventricle are subject to some variation. In the Lepidopleuridae, 
the Mopalidae, and Yonicella, Trachydermon, Boreochiton, etc., among 
the Ischnochitonidae, the auriculo-ventricular aperture on either 
side is single and anterior (Fig. 27); in the Acanthochitonidae, the 
Cryptoplacidae, and the Chitonidae generally there are two apertures 
on either side (Figs. 4 and 26); in Chiton squamosus there are three, 
and in Chiton goodalli four. The auricles are united to one another 
posteriorly, but there is never a posterior median auriculo-ventricular 
opening. The posterior auriculo-ventricular orifices are frequently 
asymmetrical. A single anterior aorta rises from the ventricle 
anteriorly and carries the blood to the various organs and inter- 
visceral blood-spaces. The venous blood from the different parts of 
the body is conducted back to a large sinus on either side near to 
the line of union of the mantle with the body. Closely connected 
with this same line of union on either side is a row of gills, situated 
between the foot and the mantle. The'number of pairs of gills 
varies from four in Lepidopleurus pagenstecheri to eighty in Acantho- 
pleura spiniger, but a careful 
comparison shows that the 
number of gills in the right 
hand row does not always 
correspond with that in the 
left. 
The gill-rows may be 
either of the holobranchial 
type, in which case they ex- 
tend over the whole length 
of the body, or of the mero- Migs fe 
branchial type, ip which they, Psst of Movie, dors! spect, 1, auricle 
are confined to a more or auricle; IV and V, afferent vessels ; VI, mantle ; 6, 7, 
ay ae 8, indicate the position of the sixth, seventh, and 
less limited space at the pos- eighth shell-plates. 
terior end of the body (Fig, 
28). But these two types are connected by transitional forms, and 
they are not, generally speaking, characteristic of natural groups, nor 
are they determined by the greater or less size of the species. The 
genera with few gills are naturally merobranchial, and in the Lepido- 
pleuridae these organs are confined to the region covered by the 
two last shell-valves. The gills are inserted at the bottom of the 
pallial groove. The largest gill, which is also the last one in those 
forms in which no adanal gills are present, is always the first 
