42 REPORT OF NEW JERSEY STATE MUSEUM. 



a relatively immense ascidian or sea-squirt on its back, and in 

 another species of the same genus the hinder leg-s are used to 

 hold umbrella-wise over the back a single valve of a bivalve 

 shell. 



Several instances of commensalism occur. Certain sea-anem- 

 ones (Adamsia) are always found on a univalve shell, such 

 as that of a whelk, and the latter inhabited by a hermit-crab. 

 The sea-anemone is carried about by the crab and in this way 

 secures a more varied and abundant food-supply than would fall 

 to its lot if it remained in one place. On the other hand the crab 

 is protected from the attack of predaceous fishes by retreating 

 into its shell and leaving exposed the sea-anemone, which, owing 

 tO' its toughness, and toi the pain caused by its poisonous stinging- 

 capsules, is usually avoided as an article of food. Other inter- 

 esting occurrences are the little oyster-crabs {Pinnotheres) liv- 

 ing in the mantle-cavities of oysters and mussels. Other deca- 

 pods are found in the intestines of sea-urchins and holothurians, 

 and one genus of crab lives in the cavity in a coral, the aperture 

 being only just sufficient to allow of a due supply of food and 

 water. 



The Crustacea apparently exhibit certain characters with the 

 object of attracting the opposite sex. The immiensely enlarged 

 and highly colored chelipeds of some male crabs (Uca) are said 

 to be used for attracting the female as well as for fighting. The 

 sound-producing organs of some decapods have probably also a 

 sexual significance. The rock lobster (Palinnrus) has a soft, 

 chitinous pad on the antenna, which it rubs against a projecting 

 keel on the sternal region of the head, producing a peculiar creak- 

 ing sound, and another macruran (Alpheiis) makes noises by 

 clapping together the fixed and movable fingers of its large 

 chelipeds. The fact that these sounds can be produced at the 

 will of the animals seem® to show that they undoubtedly possess 

 a sense of hearing, and that the auditory sac is not merely an 

 organ O'f the sense of direction. 



