27 



Just behind the eye and forming the dorsal i)Ortion of tlie side of the head 

 is a sinuous crest, the olfactory crest, which projects outward and backward 

 toward the collar so that water entering the mantle chamber passes over it. 

 The crest forms the dorsal boundary of two concavities , in the lower of which 

 is a pit lined by a sensory epithelium in which the "olfactory nerve" ends. 



The pallial chamber, the vis(;eral mass, and the pai-ijal complex. 



The mantle is essentially a fold which arises from the dorsal and anterior 

 surface of the visceral dome and extends forward around the sides of the body , 

 thus enclosing a space , the mantle chamber , which is largest below. The 

 anterior and dorsal limits of the space are determined by the attachment of the 

 mantle to the viscei'al mass. Anteriorly , the mantle chamber reaches up to the 

 dorsal end of the nuchal cartilage. Thence , the antero-dorsal boundary of the 

 cavity follows the edge of the pen to the dorsal point of the mantle , thus 

 passing obliquely aci'oss the side of the visceral mass. The cavity is lined by a 

 sheet of integument which covers the inner side of the mantle and is reflected 

 over the visceral mass. This integument forms a median mesentery which di- 

 vides the dorsal part of the cavity and supports the median mantle artery. 



The visceral mass or body is attached to the lower surface of the pen and 

 is also loosely attached to the collar by means of the sliding joints between the 

 pallial and siphonal cartilages and between the nuchal cartilage and the pen. It 

 is divided obliquely by the siphonal retractors into a dorsal and a ventral por- 

 tion. Its ventral portion lies above and in front of the siphon and siphonal 

 retractors. It consists of the liver and tlie liver case which , as described above, 

 is formed by the cephalic and nuchal retractors and the postcephalic and nuchal 

 cartilages. The liver case is conical , its broad ventral end being attached to 

 the skull and its somewhat flattened dorsal end being attached to the middle of 

 the pen between the insertions of the siphonal retractors. The siphon is attached 

 to the lower surface , and the lateral siphonal vah'es to the sides of the liver 

 case behind the neck. The siphonal retractors are attached to the case by loose 

 connective tissue and li\' the overlying integument. 



The dorsal portion of the body contains all the visceral organs except the 

 liver, it is covered above by the pen and below by the body wall which is 

 thin and transparent and is formed by the integument and the peritoneum. 



The organs of the pallial complex are grouped upon the lower surface and 

 sides of the visceral mass near the bases of the siphonal retractors which not 

 only separate the liver from the other viscera , but also divide the fore half of 

 the mantle chamber into a median cloacal , and a pair of lateral respiratory 



