CIRCULATION SENSES. 



17 



and one or two auricles (o) 

 is found in the track of the 

 arterial blood, and sends 

 this liquid to all parts of 

 the body, from which it re- 

 turns to the organs of respi- 

 ration. Sometimes we also r 

 find, at the base of the ves- 

 sels which enter this latter 

 apparatus, venous reser- 

 voirs, erroneously called pul- 

 monary hearts ; for, having 

 nothing in their structure re- 

 sembling muscle, they can- 

 not set the blood in motion, 

 and consequently do not me- 

 rit this name. (See Jig. 9, 

 Page 21.) 



13. The organs of respira- 

 tion vary so much that they 

 cannot be well described at 

 this place : we will merely 

 mention that they are some- 

 times in the form of lungs, 

 and at others, in that of 

 branchiae or gills. 



14. Nor can we say any thing generally of the structure of 

 the organs of the senses, which are, however, always less com- 

 plete than in the vertebrate animals. Certain mollusks appear to 

 be endowed only with the sense of touch and with the sense of taste, 

 but in a great many we find eyes which vary in structure, and 

 in some of these animals there even exists an apparatus of hear- 

 ing : but none is known to possess a particular organ of smell. 



15. Mollusks are re-produced from eggs; but are sometimes 

 hatched without, and sometimes within the body of the mother, 



Fig. 6. HEART OF AN ACEPHALOUS 



3IOLLUSK. 



Explanation of Fig- 6. Heart of an acephalous mollusk : v. the ventri- 

 cle, which in these animals has the rectum (r) passing through it, and 

 gives rise to two arteries (a) ; c. one of the auriculo-ventricular openings ; 

 o. the two auricles receiving the blood from the branchial veins (&) 



In the above diagram, the ventricle (?>) is open, and the rectum, (r, r) 

 has been divided, and the lower part turned downwards : a (at the lower 

 part of the figure) represents the origin of one artery, the other artery 

 having a similar origin above. 



1 3. What is the character of the organs of respiration in mollusks ? 



14. What senses are possessed by mollusks ? 



15. How are mollusks re-produced ? 



2* 



