NUTRITION 99 



holothurians. Allen observed that the oesophagus of a species 

 of Syllid pulsates at a rate of 250 per minute. In most cases 

 rhythmical movement can be induced by applying gentle 

 stretcliing, e.g. by attaching a strip of gut to a light lever. 

 This can be shown in the crop of Helix (Ten Gate), Aplysia 

 (Brucke), the rectum of Astacus (Ten Gate), or in ring prepara- 

 tions of the pharyngeal musculature of Aphrodite (Hogben 

 and Hobson). The property very commonly displayed by 

 plain muscle in responding to gentle stretching by contraction 

 is probably very important in the production of churning 

 movement in the gut. In Lamellibranchs there is very little 

 muscular tissue associated with the alimentary tract, which 

 is ciliated throughout ; and ciliary movement is the main 

 factor in propelHng the food from the mouth to the anus in 

 these animals. In many LameUibranchs, however, the rectum 

 pierces the ventricle, and by inserting a cannula in the former. 

 Ten Gate (1924) has shown that the pressure in the rectum 

 of Anodon undergoes rhythmical variation in unison with the 

 heart beat. It seems that the heart may here function as a 

 means of promoting evacuation of rectal contents. The 

 nervous control of defaecation has been studied in the lobster 

 (Homarus) by Miller (191 2). The radial musculature that 

 controls the closure of the anus (there is no true sphincter) 

 is suppHed by fibres from the last abdominal ganglion, and 

 stimulation of these nerves produces rhythmical defaecation 

 movements (incomplete tetanus). It is doubtful whether the 

 mechanism of defaecation is simply a segmental reflex, since 

 such movements can be induced after section of the nerves. 



There has been little important work relating to the 

 physico-chemical aspects of absorption based on invertebrate 

 material. However, two points are worth mentioning. The 

 phenomenon of intracellular digestion, familiar enough in the 

 case of coelenterates, is much more widely prevalent in the 

 invertebrate phyla than is generally recognised. It occurs 

 in animals with very elaborate digestive systems such as 

 Gasteropods and Lamellibranchs. Yonge has photographed 

 ingested diatoms in the cells lining the stomach of the bivalve, 

 Mya. 



