204 ZOOLOGY 



blood that is forced out of the ventricle finds almost immedi- 

 ately two paths open to it ; one large vessel {post, aort) goes 

 to the digestive gland, the intestine, and part of the repro- 

 ductive organs, while the other vessel {ant. aort) leads to the 

 main muscles of the head and foot, the stomach and crop, and 

 to such of the reproductive organs as are not supplied by 

 the first artery. Over each organ the artery breaks up into 

 the finest capiUaries so as to come into most intimate contact 

 with the tissues, where it does its work of receiving or convey- 

 ing fluid food and oxygen. Beyond, these capillaries flow 

 together into veins by which the blood is carried to the 

 mantle chamber or lung. Here is another capillary system 

 in which the blood is aerated and out of which it is collected 

 into a large vein that leads directly to the auricle of the heart. 

 The blood of the snail, as of most other moUusks, is color- 

 less. However, on exposure to the air, so that it can absorb 

 oxygen, it turns slightly bluish. This change of color is due 

 to the oxidation of a substance that carries oxygen from the 

 respiratory surface to the tissues, just as the red coloring matter 

 (hem'oglobin) of our blood does. The oxygen carrier of 

 mollusks is called hem'ocyanin. A few mollusks have also 

 hemoglobin in the blood. 



Fig. 196. — Dissection of a large marine snail, Triton. The mantle cut along 

 dorsal line leaving gill on left flap and rectum on right, pharynx, crop, and 

 kidney cut into to show structure, an, anus; aiil. aort, arterj- going to 

 head ; aur, auricle (receiving chamber) of heart ; hue, cavity of pharjmx ; 

 cer. hue. con, nerve connecting pharynx to brain icer. g) ; cten, gill ; int. intes- 

 tine ; I. hue. 0, left pharyngeal ganglia ; I. sal. gl, left salivary gland ; neph, 

 kidney and its opening (neph. ap) ; od, odontophore ; OfS, cesophagus, cut at 

 oes' ; osph, " osphradium," a taste organ: avid, o^aduct ; ■pluer. {/, ganglion 

 supplying the ^dscera : post, aort, artery to \'iscera ; post, oes, part of cesoph- 

 agus behind crop ; rad. s, radula sack : reet, rectum ; sal. du, salivary 

 duct ; .nph, siphon ; supr. g, ganglia above \-iscera ; tent, tentacle and its 

 nerve, tent, n ; vent, ventricle of heart. 



