44 AMERICAN JOURNAL 



cases among the lamellibranchs we find two distinct hearts.* 

 The young conchifer is attached in the beginning by a byssiform 

 peduncle, is without latnelliform gills, and respires by the vascu- 

 lar and ciliated surface of the mantle lobes, and has two distinct 

 hearts. f They are sometimes furnished with setae or bristles 

 (speischen) upon the mantle lobes in their larval condition, as in 

 Anodon. 



The gap between the conchifers and gasteropods presents no 

 well marked interruptions, though the Dentaliidce and some ptero- 

 pods, perhaps come nearer than any others toward bridging it. 



The genitalia, respiratory apparatus, embryology and the 

 curves of the alimentary canal present such differences and modi- 

 fications that it is impossible to generalize briefly upon them in 

 this sub-division. 



The blood is sometimes colored, but more generally translu- 

 cent. 



The oral aperture and efferent orifice (when the latter exists, 

 as it usually does) are, in the majority of cases, contiguous. 



Striated muscular fiber has not yet been detected in the cepha- 

 lopods, but is known to exist in members of the other two 

 classes. J 



Many conchifers are attached by a more or less permanent 

 byssus, and some gasteropods, as well as conchifers, are attached 

 by the calcareous matter of their external shells, to which they 

 are organically fixed by muscles. 



One group among the gasteropods merits special attention, 

 on account of the many points in which it offers some analogy 

 with articulate animals. I refer to the Polyplacophora or chitons. 

 This remarkable order, in its dorsal vessels and (in some cases) 

 fasciculated setae, resembles the annelids ; in its differentiated 

 carapax-like head (in the young condition) and successive hard 

 plates recalls the Crustacea ; while its symmetrical genitalia and 

 lateral branchiae bear a distant relation to those of the conchifers. 

 It is to be observed, however, that the apparent segmentation 

 is confined to the valves and fasciculated setae, and that the in- 

 ternal organs present no similar series of repetitions. The head 

 and foot, odontophore, nervous system and alimentary canal are 

 typically molluscan, and of the balance of characters there can 

 be no doubt. The eyes, present in the young, are lost in the 

 adult condition ; the pores of the girdle, though perhaps not 

 homologous, present some analogies with the perforations of the 



* Huxley, Intr. Class, p. 34, 1869. 

 fOwen, Lect. Comp. Anat. Inv.p. 528. 



Dall On striated muscular fiber in the Gasteropoda, Silliraan's Am. 

 Journ. Sci. and Arts, Feb. 1871, p. 123. 



