170 



MORSE, CLASSIFICATION OF MOLLUSCA 



ogies amongthe Articulata ; Cirripedia, for example, where 

 we have animals becoming attached bead downward, and 

 all the oral parts, as in the pedunculated forms, tending 

 towards the posterior pole of the body ; or in Limulus, 

 where we have such a decephalization as it were, the mouth 

 occupies nearly a central position in the ventral region. 

 Again, considering the intestine as a simple tube, open- 

 ing at each end, with the weight of structure evenly divi- 

 ded between the two openings, is it any more incredulous, 

 that the oral opening should be posterior, than that the 

 anal opening should be anterior, as in the Gasteropods ? 



In Polyzoa, the oral and anal openings occupy a simi- 

 lar position in all the forms. In Brachiopods, while the 

 mouth remains in nearly a constant position, the anus 

 terminates either in a median line, or by a lateral deflection 

 of intestine to one side. In Tunicata, while the mouth oc- 

 cupies a permanent position at the front of the sac, the 

 anus terminates at various portions of the sac, generally in 

 a median line, though there is usually a lateral deflection of 

 the intestine. 



In Lamellibranchiata, the mouth and anus terminate 

 in a median line, with few exceptions, (e. g. Pecten) though 

 the intestine convolutes in various ways. In Gasteropods 

 we have again lateral deflection of intestine, and though in 

 many genera the anus terminates in a median line, yet in 

 the bulk of the Gasteropods it terminates at one side or 

 the other. In the Dibranchiate Cephalopods we have 

 again the termination of the intestine in a median line. 



o The diagram here given (Fig. 1) re- 

 presents an ideal longitudinal section of 

 the sac, similar to those of Series I. The 

 arrow within the sac, shows the direction 

 of rotation of the bent intestine,- carrying 

 with it the heart, (see Plate, Series I.) 

 which in Brachiopoda we find on the ven- 

 tral region ; in Tunicata on the anterior 

 dorsal region ; in Lamellibranchiata on 

 the dorsal region ; in Gasteropoda on the 

 dorsal region and also further back ; and 

 in the Cephalopods at the posterior portion of sac. The 

 different positions of the sac openings (represented in fig. 



Fisr. 1. 



