PHYSICAL FEATURES OF THE IRISH SEA. II 



3. Depths of over 20 fathoms are only found to the west, north, and south of the 

 Isle of Man ; and depths of from 20 to 50 fathoms give us the most varied bottom 

 deposits and the richest fauna. As a rule, the sand is more or less mixed with mud, 

 and, as the bottom goes deeper, the amount of mud gets greater. When there is a 

 considerable admixture of mud with coarse sand it forms what is known to the trawlers as a 

 "reamy" bottom, and that is the ground upon which the sole and some other fish are 

 generally found spawning. 



Shells and other hard parts of animals play an important part in the deposits at depths 

 of about 20 fathoms and upwards. In places the dredge comes up filled with Pecten shells, 

 dead and alive, chiefly P. opercularis and P. maximus. At other places the deposit is practi- 

 cally composed of the shells of Pectunculus glycimeris. These and other shell beds form a rich 

 collecting ground to the naturalist, as they support an abundant and varied fauna. Zoophytes 

 and polyzoa are attached to the shells, and these serve as shelter for nudibranchs and other 

 small mollusca, worms, and ascidians. On the whole the heterogeneous deposits support a 

 richer fauna than do the homogeneous deposits, such as sand or mud, and it is chiefly in the 

 -/.one of depth we are now considering that the heterogeneous deposits occur. 



4. The depths over 50 fathoms contain a pure dark bluish-grey mud, which is very 

 tenacious, and sets when dried into a firm clay. This is abominable stuff to dredge in and to 

 work with on deck. It clings to everything that touches it ; it is almost impossible to see what 

 is in it, and to get the animals out of it uninjured ; it is too solid for the sieves, and the hose 

 can be played upon masses of it almost indefinitely without dissolving it. The fauna of this 

 zone is, in our district, quite peculiar and characteristic. In its shallower parts, about 50 

 fathoms, it contains great numbers of living and dead Turritclla terebra, upon many of which 

 are attached one, two, or three specimens of the little red anemone, Sagartia cxpansa. In its 

 deeper parts, up to 80 fathoms, are found Calocaris macandrece, Hyalincecia titbicola, a small 

 LTimbriconereis , Pan/halt's oerstedi, Li pob r an chins Jeffrey sii, Brissopsislyrifera, Amphiura chiajii , 

 and Isocardia cor. Numbers of large sausage-like muddy tubes, formed of stratified layers of 

 interlacing threads of mucus in which the mud particles are closely entangled, are brought up in 

 the dredge. These we have proved to be the tubes of Panthalis oerstedi, and the living annelid 

 has several times been found in the tubes, but most of those we dredge up are empty, and the 

 tubes are certainly far more numerous than the worms. Apparently the explanation is that the 

 Panthalis forms a tube as it lies in the mud, and then when it moves away leaves its tube 

 behind it (one can scarcely .imagine the animal dragging such a tube through this tenacious 

 deposit), and after a time forms another in a new situation. 



These are the leading conclusions we have come to so far in regard to the distribution 

 of sub-marine deposits in our area. Two further questions now present themselves : first, the 

 biological one the effect upon the fauna ; and secondly, the geological one the origin of the 

 deposits. In regard to the importance of the nature of the bottom to the animals living upon 

 it there can be no doubt. Probably the nature of the deposit is the most important of the 

 various factors that determine the distribution of animals over the sea-bottom within one 

 zoological area. It is certainly more important than mere depth ; a muddy bottom will 

 support a similar fauna at 10 fathoms in one place and at 50 fathoms in another. Probably 

 the most important influence in the environment of a lower animal is its food, and once 

 beyond the narrow sub-littoral zone in which algae flourish and to which, of course, certain 

 phytivorous animals must be restricted it is probably chiefly the nature of the bottom which 



