A. K Yerrill — The Omsthoteuthidm. 77 



strained, and as I believe, erroneous views of the orientation 

 and homologies of the parts of cephalopods advanced by Hux- 

 ley and still held by many other authors. 



Thus they say that " the arched superior or posterior surface 

 includes not only the posterior aspect of an ordinary cephah 

 opod body but also the dorsal and ventral surfaces, as well as 

 the outer surface of the umbrella. The head and body are 

 thus flattened antero-posteriorly." According to their view, 

 the siphon is " directed in the ventral, not in anterior direc- 

 tion, as is usually the case." They consider that " the ventral 

 portion of the body is nowhere to be considered as concealed, 

 except perhaps the mantle-rim, which is reflected inward at 

 the branchial aperture. On the contrary, it stands, at least for 

 the greater part, exposed more than ever, only with this pecu- 

 liarity, that it forms a part of the general superior surface, 

 thereby losing all definable boundaries from the real dorsal 

 region or from the outer surface of the umbrella. Also that 

 the original posterior end " is to be sought somewhere near the 

 middle of the upper surface of the flattened body, behind the 

 dorsal cartilage." 



JFiGURE 5. — The same as Fig. 3. Posterior view of gill cavity; 6,&, gills; A, 

 branchial heart ; 5, septum of gill cavity ; 5t, siphon ; a, anus ; v, sperm duct. 



The fact that such very unnatural and strained interpreta- 

 tions of the relations of parts have become necessary, ought to 

 be sufficient to show that the theories on which they are based 

 are erroneous. In fact, much of the interest connected with 

 these forms consists in the fact that they throw much light on 

 this very question of the relation of parts in the cephalopod 

 body. 



My own view of molluscan polarity is that the rudimentary 

 cephalic ganglion which appears in the middle of the apical 

 plate of the gastropod or bivalve larvge, indicates the true 

 anterior pole, while the mouth is behind the ventral edge of the 

 apical plate, as in annelid larvae. Later, by the progress of growth, 

 the apical plate is reduced in size and the cephalic ganglions 

 become more closely approximated to the mouth and oesopha- 

 gus, so that the mouth may be considered as practically close 

 to the anterior pole. The shell-gland, I think, may be taken to 



