XVII VERTICAL DISTRIBUTION 



485 



but when they are found they usually occur in great numbers. 

 During the cruise of the Cliallengei\ dredging was conducted at 

 361 stations ; at only 38 or 39 of these were Brachiopoda brought 

 up. Mr. Cuming, quoted by Davidson, records that after a great 

 storm in the year 1836, he collected as many as 20 bushels of 

 Lingula anatifera on the sea-shore at Manilla, where, he relates, 

 they are used as an article of food. It has been suggested above 

 that their abundance in certain localities is due to their limited 

 powers of locomotion, which are effective but for a few hours, 

 the larva being, moreover, so minute that unless borne by a 

 current it could not travel far from its parent. When once 

 settled down it has little to fear from the attacks of other 

 animals. The size of its shell relative to its body would deter 

 most animals from regarding it as a desirable article of food, 

 and as far as is known at present the Brachiopoda suffer but 

 little from internal parasites, the only case I know being a 

 minute parasitic Copepod belonging to a new and as yet unnamed 

 genus which I found within the mantle cavity of Cistella (^Argi- 

 ope) neapolitana in Naples. Their slight value as an article of 

 diet has doubtless helped to preserve them through the long 

 periods of geological time, through which they have existed 

 apparently unchanged. 



Two of the recent genera of the family Lingulidae, Lingula 

 and Crlottidia^ are usually found between tide-marks or in shallow 

 water not exceeding 17 fathoms. Discina is also found about 

 the low-tide level, but one species at any rate, Discinisca atlan- 

 tiea^ has been dredged, according to Davidson, "at depths rang- 

 ing from 690 to nearly 2425 fathoms." Their larvae frequently 

 settle on tlie shells of their parents, and thus numbers of over- 

 lapping shells are found clustered together. Crania is usually 

 dredged from moderate depths down to 808 fathoms, adhering 

 to rocks, lumps of coral, stones, and shells. 



Of the Testicardines, Terebratula Wyvillei has probably been 

 found at the greatest depth, i.e. 2900 fathoms, in the North 

 Pacific. It is interesting to note that its shell is glassy and 

 extremely thin. The Brachiopoda are, however, as a rule, 

 found in shallower water; they abound up to a depth of 500 or 

 600 fathoms, after which they rapidly diminish with increasing 

 depth. About one-half the named species occur at a depth of 

 less than 100 fathoms. 



