160 



ZOOLOGY 



shells, protrude their elongated appendages, which together 

 form a sort of rake, and pull in particles which happen to l^e 

 floating aljout them. Other species of barnacles attach them- 

 selves to floating seaweed, ship l)ottoms, and whales; under 

 these circumstances, despite their sessile hal)it, the}' enjoy 

 a constant change of locality. Barnacles doubtless gain great 

 protection from the circumstance that they are sessile and 

 enclosed in shells ; but their peculiar habits have given rise to 

 certain peculiarities in reproduction. They are hermaphro- 

 ditic ; i.e. both male and female germ cells occur in the same 

 individual. Despite this fact, dwarf male individuals are 

 occasionally found inside the shell of the barnacle ; these are 

 known as " complemental males." The general form of the 



barnacles has also become gTeatly 

 modified by their sessile habit, 

 so that they were long regarded 

 as mollusks, until it was shown 

 that the larva^ are almost exactly 

 like those of other Entomostraca. 

 Trilobites ^ are extinct giant 

 Entomostraca, closely allied to 

 Branchiopods. They were im- 

 mensely aliundant in early geo- 

 logic times, and their remains 

 form a large part of certain rocks. 

 The}' had a segmented boch', with 

 bifid appendages and long an- 

 tenna, and their compound eyes 

 were liorne on the great frontal 

 shield. Some of them were nearly 

 half a metre long (Fig. 158). 



^ Ha\-ing three lobsters. 



Fig. 1.5S. — a restoration of the 

 ventral asijeet of a Trilobite. 

 Note in particular the character 

 of the appendages. After Beechcr. 



