230 CRUSTACEANS. 



It is a creature very easily recognised, however, from the form of its 

 body, and of the larger or external antennae. 



The finest specimens extend above eighteen lines from the tip of 

 the antennae to the lower extremity. The body subcylindrical, is com- 

 posed of nine segments, whereof the first and last are the head and tail, 

 the latter much exceeding the length of any of the others. Four an- 

 tennae, two larger and two smaller, are borne on the head, the larger or 

 exterior pair consisting of five articulations, with a large black eye seated 

 at the root of each. Fourteen feet, armed with claws, are disposed in 

 seven pair along the body, whereon some bristles are to be seen. Colour 

 greenish. 



This is an active creature, it darts swiftly through the water, some- 

 times tumbling amidst it. When swimming, the larger antennae are 

 closed, and advanced in front. 



It feeds readily and greedily on animal substances, devouring them 

 also as they are carried along between the fore legs. 



PLATE LXIII. 



Fia. 10. Oniscus (Ideotea) entomon, enlarged; fig. 10, a, relative proportion 

 between the body and larger antennae. 



6. AsELLtJS MEDUSAE. Plate LXII. figs. 6, 7. 



Two parasites, which I can describe but imperfectly, frequent the 

 Medusa ; though whether to prey on it, or merely for rest and pro- 

 tection, I have been unable to ascertain. 



Their appearance is very different from any of the preceding ani- 

 mals ; and unless in the presence of two eyes, both bear much resem- 

 blance to the Cyclopean race among the Entomostraca. 



Length about half an inch ; thorax large and thick, occupying 

 half the whole extent, the lower part of the body tapering abruptly 

 downwards, while small in proportion. 



Two short spinous black tipt antennce, and other two are below, some- 

 what apart from them, of about the same length. Two large oval eyes 



