INVERTEBRATE BOTTOM FAUNA 479 



though the conditions vary in different years. Below 2 metres 

 the summer temperature decreases as we approach the bottom, 

 but late in autumn and in winter the temperature is highest at 

 the bottom. 



In the intermediate warm salt water layers we get a fauna 

 abounding in individuals that form a distinctive feature of the 

 pools. There is, first of all, the oyster, Ostrea edtdis, which 

 finds its principal home here, and there are also quantities of 

 Pecten operailaris attached to the rocks all round. The 

 ascidian fauna is represented by several species, which are all 

 exceedingly plentiful, the commonest being Ascidia vientttla, 

 Ascidiella aspersa, Ciona intestinalis, and Clavellina lepadiformis} 

 The most abundant of the bryozoans is Aetea, while a species 

 of Botigainvillia appears to be the commonest hydroid. The 

 principal sea - anemones are Metridium diantJms, Urticina 

 crassicornis, and a species of Sagartia. Parechinus miliaris is 

 the only echinoid, but it occurs in great numbers. Ostrea, 

 Pecten, and Pai'ecJiinus indicate the decidedly southern 

 character of the fauna, and it may not be out of place to 

 mention that among the plankton forms we get a copepod 

 {Paracartia grani) belonging to a genus not met with again till 

 we reach the west coast of Africa. 



In addition to the forms having a southern distribution and 

 of southern origin, however, we find eurythermal and euryhaline 

 forms. Asterias rubens, Carcinus i?icenas, and Mytilits edzilis 

 are nearly always present, the last named in particular being in 

 great abundance, frequently attached to the lines stretched 

 across the oyster-pools for carrying the bundles of twigs or the 

 baskets to which the oyster spat attaches itself. Mingled with 

 this assemblage of mussels, ascidians, etc., we get enormous 

 quantities of smaller animal forms, the crustacean family Tanaidae 

 being invariably represented. 



Among the forms described as characteristic of the littoral vertical 

 zone, there are very few that do not occur in all its depths, that fije^iuorar °^ 

 is to say, only a few forms are restricted to the actual strand- fauna. 

 belt. These few, however, include most of the forms that 

 characterise the tidal area." No doubt even these may 

 occasionally be met with at a depth of a few fathoms, but 



1 In enclosed places, though not actually in pools, Corclla paralldogya»ima is also common. 



- For instance, Patella vulgata, Piu-pura lapillus, Littorlna littorea, L. rudis, and L. obtiisata ; 

 besides Balanus balanoides, Mytilus edulis, Oixhestia littorea, Campaiiiilaria flexiiosa, Clava 

 squatjiafa. Actinia equina, Alcyoniditwi hirsutum ; and among the burrowing species Alya 

 arenaria, Carditim edtile, and Arenicola piscatornm. 



