254 



PROF. W. A. HEHDMAN ON THE ABUNDANCE 



or when populated under favourable conditions as on a mussel scalp, seems 

 to be more productive than the land *. 



Styelopsis. 

 One of the most densely-covered rock surfaces that I have seen is in the 

 large caves near Spanish Head at the south end of the Isle of Man. These 

 caves have to be entered by boat at low tide, and are of considerable extent, 

 and their walls between tide-marks and as far down as can be seen in the 

 clear deep water are covered with an assemblage of animals, consisting of: 

 the Tetractinellid sponges Pachymatisma johnstoni and Stelletta collingsi, 

 several species of small white calcareous sponges, HalicJiondria panicea, 

 Tiihularia indivisa, Sagartia miniata, and enormous quantities of the small 

 red Ascidian Styelopsis grossularia. The Ascidians cover by far the greater 

 part of the rock, and are so closely placed that their bases are continuous, 

 and masses can be peeled off in small sheets (fig. 6). On the average of a 



Fig. 0. — Two aggregations of Styelopsis grossularia, nat. size. 



number of measurements they are three-quarters of an inch in height, and 

 there are from 10 to 30 in the square inch according to size, over 50,000 to 

 the square yard. 



Although Styelopsis is not generally supposed to be a common animal, the 

 numbers present in these caves, and no doubt in many other similar places 

 on rocky shores, must be very large, and must afford a very considerable 

 amount of highly nutritious food. In many parts of the world Ascidians 

 closely related to our Styelopsis (0ynthiada3) are used as food by man. 



Other Common Shore Animals. 

 It may be of interest to add here the resrdts of countings of a few other 

 common shore animals, although I shall not discuss them further. Along 



* As Spenser seems to have known, or imagined, more than four hundred years ago : — 

 " O what an endlesse worke have I in hand, 

 To count the seas abundant progeny, 

 Whose fruitful seede farre passeth those in land," &c— 



Faerie Queen, Bk. IV. Canto 12. 



