138 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



similarly named part in the Yertebrata ; and, indeed, 

 where best studied, they have been found to have 

 rather the function of the pancreas. In the dibran- 

 chiate Cephalopoda a rectal caecum secretes an inky 

 fluid, which was formerly used for writing and for 

 the manufacture of sepia ; this is the so-called ink- 

 bag. The secretions of the Octopus have been found 

 to be all acid. 



In the Lamellibranchiata (or Acephala) the 

 odontophore is completely absent ; the intestinal tract 

 is comparatively simple, but varies in the extent of 

 its convolutions ; in its walls, or in an appended 

 caecum, is the so-called crystalline style or stalk, a 

 transparent rod-like structure of unknown function. 

 Its absorbent surface is sometimes increased by the 

 development of a typhlosole, as in the earthworm, 

 and the terminal portion very frequently passes 

 through the dorsally-placed heart. 



In all Chordata we observe that, as also in 

 Balanoglossus, the anterior posterior of the diges- 

 tive tract is primarily divisible into an upper and a 

 lower portion, one of which serves as the means of 

 passage for the water of respiration, and the other as 

 the food passage. Postponing for the moment (see 

 page 231) the consideration of the former, and insist- 

 ing only on the significance of this arrangement as a 

 leading point in the morphology of the Chordata, 

 we observe that in the Tunicata the exclusively 

 nutrient region of the enteric tract commences at 

 the bottom of the respiratory part by a rounded 

 or funnel-shaped opening; the tube, which varies in 

 calibre in different parts, is often looped, and in 

 such cases the anus comes to lie not far from the 

 mouth. 



Among the Cliordata we find very simple 

 arrangements of the digestive tract in the Cepha- 

 locliordata ; the mouth of the Lancelet is placed 



