THE-AMPHJNEURA 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 Tonicella, 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 Acantlio- 

 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- 



Vkranr-Viifll t\rr\p in wVnVVi thpv Heart of Mopalia, dorsal aspect. I, auricnlo- 



Drancmal type, m wnicn tney ventricular communication ; II, ventricle; III, right 



are Confined tO a more Or auricle ; IV and V, afferent vessels ; VI, mantle ; 6, 7, 



, . . , 8, indicate the position of the sixth, seventh, and 



less limited space at the pos- eighth sheii-piates. 

 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 



