THE DIGESTIVE TRACT OF ELASMOBRANCHS iff 
Physiologically active hydrochloric acid by AgNO,=C: 
0. 660 
. 525 
Action of the gastric juice on chitin.—Lobsters and crabs form part of the food 
of several of the elasmobranchs. The shell of these crustaceous consists of chitin and 
salts. This chitin is very resistant to reagents. According to Hammarsten (1901), 
chitin, to which he gives the formula C©,,H,,)N,O,,+-n(H,O), is insoluble in boiling 
water, alcohol, ether, acetic acid, dilute mineral acids, and dilute alkalies. It is 
dissolved without decomposing in cold concentrated hydrochloric acid. Since chitin 
is so resistant it is interesting to know whether the chitin-eating fish digest chitin or 
whether it passes through the body unchanged. 
An analysis of lobster shells given by Herrick (1895) from the work of Albert 
W. Smith is as follows: 
TaBLe SHOwriNG CompositioN OF THE CARAPACE OF THE LopsTER (3 SPECIMENS). 
Composition, air dried. u De 3. 
Weightin’grams..-.--..2:..<.<.-- wiitels «nk eles - Sateeows so kame eee me ame ehinn Cos Re ees nee 
Calculated as calcium carbonate ......-- Se eR OE OOO Saas ee 
Calcium phosphate .......--..-.. Pade oeeemee sade oonca eee edeoees 
Galéium/ sulphate. .<-........2.2.....: S BE RRS Me Peseta A hes Se OR NED 5 ta noe 
Magnesium carbonate ......-..........----...- Sethe ee eee eee eee 
Sodium carbonate .-- : 
Alumina ........-. 
Silica xa Be ae Ri tie ae ak aaa AE pene ee ed ae ae tee eae ek eee Ores 
Organic matterand w Elta ate ee ene Sin ae a Sa A oe Ce SRA ES oS seine ae mere 
Richet (1878) believed that the chitin of the shell of crabs, lobsters, ete., is 
digested by dogfishes and sharks, although he recognized that it is extremely diffi- 
cult to dissolve chitin by artificial digestion. Yung (1899), on the other hand, claimed 
that the selachians do not digest chitin, for he found pieces of chitin not only in the 
stomach but even in the spiral valve and rectum. More recently Zaitschek (1904) has 
proved quantitatively that the chitin in the wings of insects is absolutely undigested 
by hens. 
To determine whether the elasmobranchs, and especially the smooth dogfish, 
digest chitin, the following experiments were made: 
1. Several fishes were fed with crabs and lobsters and in the course of from one 
to five days were killed. In some cases the shells were found in the stomach ina 
macerated state. On the other hand, no compact chitin could be found after ninety 
hours of digestion, but in the spiral valve might be found a gritty dark-brown or 
reddish mud. 
2. The gastric juice was drawn from several large dogfish. Into small quanti- 
ties of this juice lobster shells were placed. The mixtures were kept at a constant 
temperature—some at 18° C., some at 38° C. After twelye hours the only change 
found in the shell was that the edges were softened a trifle. 
3. The mucous membrane of the stomach of five dogfish was scraped, triturated 
in glycerin and 0.5 per cent hydrochloric acid. This juice, although it acted quickly 
on fibrin, did not digest the chitin in forty hours. 
4. Experiment 2 was repeated with the difference that at frequent intervals the 
gastric juice was renewed and the chitin was subjected to frequent grinding. In this 
