HABITS AND ECONOiMY OF THE MOLLUSCA. 143 



Nematoid worms are also parasitic in the Limaces. Some of 

 the HiruclineaB attack shell-fish ; Malacobdella lives upon Mya, 

 Yenus, Cyprina, Cytherea ; Clepsina, Aulastomum, Nephelis, 

 etc., attach themselves to the tissues of fresh-water mollusks. 

 Dr. Leidy has recently described a larval Distoma cornifrons, 

 found in the liver of Donaxfossor, and which probably undergoes 

 further development in some of the shore birds or fishes which 

 eat the Donax ; the same mollusk also harbors the infusorian 

 Trichodina pediculua. 



Means of Defense. In no class is the instinct of self-preser- 

 vation stronger, noi the means of defense more adequate than 

 in the moUusca; their shells seem expressly given to compensate 

 for the slowness of their movements, and the dimness of their 

 senses. The cuttle-fish escapes from attack by swimming back- 

 wards and clouding the water with an inky discharge ; and the 

 sea-hare (Aplysia) pours out, when irritated, a copious purple 

 fluid, formerly held to be poisonous. The purple fluid of the 

 Murex, Purpura and Mitra, the whitish liquid produced by the 

 marginal pores of Oncidiella, and the mucous secretion of 

 terrestrial snails are all probably protectory in character. The 

 dorsal papill* of ^olis, carry at their extremity an urticating 

 apparatus, consisting of a capsule, opening by an orifice, and 

 containing some cellules, in the interior of which is enrolled a 

 simple or barbed filament ; this apparatus is analogous to those 

 of the actiniae and many of the coelenterata. 



The prick of the lingual teeth of some mollusks is poisonous. 

 Captain Belcher was thus bitten hy a Gonus which he was 

 handling, and the slight wound was followed by a very painful 

 swelling. Gonus niarmoreus is considered a dangerous animal in 

 New Caledonia ; and accidents caused by the teeth of Gonus 

 textile are rather numerous in the New Hebrides (Montrouzier). 

 Quoy and Gaimard have remarked that the lingual teeth of Cones 

 are hollow, and provided with a sort of fish-hook at their free 

 extremit,y ; they are supposed to be detachable ; and this hypoth- 

 esis appears to be confirmed by the presence of a contraction 

 near the insertion of the tooth on the lingual plate (x, 5). 



Lamellibranchs and a large number of gastropods escape their 

 enemies by the resemblance of the color and surface of their 

 shell to their surroundings. Thus Patella and many other genera, 

 are ordinarily covered with Balani and marine growths ; others 

 are hid under Bryozoa and encrusting sea-weeds, Serpulse, Ver- 

 metidse, Alcyonaria and Sponges ; Xenophora solders shells and 

 coral to its shell, or covers it with small pieces of stone, so that 

 it looks like a small heap of dead shells or pebbles ; the Poulpe 

 (Octopus) shelters himself under the carapaces of crabs, or 

 covers himself with bivalve shells; Linja forms a retreat in a 



