MOLLUSKS 315 



In bivalves the eggs are retained within the parent shell until 

 hatched, and the young, which then escape into the water, are very 

 different-looking creatures from the parent. Generally the young 

 bivalve is free-swimming and exceedingly lively in its move- 

 ments ; sometimes by means of cilia it is enabled to attach itself 

 to any passing host, and thus the species is widely disseminated. 

 The fry soon lose their embryonic form and take on the charac- 

 teristics of the adult. 



The longevity of mollusks varies greatly with the species. Some 

 attain an age of from ten to fifteen years, while some are only 

 annuals. Oysters continue to grow for four or five years, after 

 which they may survive many years more. In truth, little is 

 known concerning the duration of life even of our better-known 

 shore species. 



In point of intelligence, mollusks must be relegated to a very 

 low position. They give evidence of possessing no more than the 

 most primary instincts, those of self-preservation and of repro- 

 duction. There are some forms that exercise a high degree of 

 protective mimicry. The highest class of mollusks, the Cephalop- 

 oda (cuttlefishes, octopi, etc.), is widely separated from the other 

 classes in the development of faculties that appear to correspond 

 with intelligence. They are exceedingly crafty in the pursuit of 

 their prey as well as in eluding capture. 



STRUCTURE OF MOLLUSKS 



Among the invertebrate animals, especially in the lower orders, 

 wide physiological departures from central or typical forms 

 are frequent. It often happens that a whole group or even sub- 

 order will differ so materially in its general characteristics from 

 another group or suborder belonging to the same phylum that the 

 student will be puzzled at first to understand how the two can be 

 nearly related. In many such cases it is only when the essen- 

 tial features of the phylum are thoroughly understood that the 

 relationships of its suborders can be appreciated. An example 

 of widely divergent groups within the same phylum has already 

 been shown in the Echinodermata. Who would at first suppose 

 that the graceful Astrophyton could bear kinship to the inert holo- 



