NUTRITION 



91 



ciliated groove formed by the distal ends of the filaments. 

 In this food - groove a strong anteriorly directed ciliary 

 current washes the mucous stream on to the labial palps, 

 whence they are propelled — still by ciliary action — into 

 the mouth (Orton, Kellogg, Yonge). There are often special 

 arrangements for excluding coarse particles, sand, etc. In 

 the primitive gastropod Crepidula — and probably other 

 marine prosobranchs — we find analogous phenomena. 

 According to Orton (1913) in Crepidula an ingoing and 

 outgoing current is established along a definite pathway 

 and the single gill acts as a strainer between them. The 

 filaments lie parallel in a horizontal line extending along the 

 left side of the mantle cavity, dividing it into a left ventro- 

 lateral inhalent chamber and a right dorsventral exhalent 

 chamber. In feeding, the front end of the shell is raised 

 slightly, water is drawn in along the anterior half of the shell 

 on the left, passed through spaces between the gill-filaments, 

 and expelled along the front half of the right edge of the shell. 

 Upon reaching the tips of the filaments, the food-particles, 

 driven along in a mucous stream by the frontal cilia, are 

 deposited in a food- groove, like that already seen in lamelli- 

 branchs, running along the right side of the body. Eventually 

 the food-masses are seized on by the radula. Ciliary feeding 

 occurs in Brachiopods, Polyzoa,and some Polychaeta. Entangle- 

 ment of food-particles in slime is also seen in small Crustacea 

 such as Daphnids, v/here the labial glands exude a stream 

 of mucilaginous secretion which entraps suspended matter in 

 the ventral current produced by the thoracic appendages, to 

 be seized on by the mouth parts. 



The History 0! the Foodstuffs.— We may now turn to con- 

 sider the changes which the three principal classes of organic 

 food-constituents undergo in the digestive tract, and their 

 subsequent fate in the body. For a detailed treatment of the 

 latter, standard monographs on biochemistry must be con- 

 sulted ; such knowledge as we possess is derived very largely 

 from clinical sources and from the study of mammalian 

 physiology. 



As proteins exist in colloidal form, they are incapable of 



