MOLLUSCS, OK SHELL-FISH. 235 



often curious and interesting in appearance, and in several 

 cases are among the commonest of the objects found on the 

 seashore. 



Among the Azygobranchs we shall begin with the familiar 

 "silver Tommies" of our youth, the "tops" of south- 

 country children, and the Trochi of scientists. All children 

 recognise at least two varieties, the common "silver 

 Tommies," valuable only in very early youth, and the large 

 pink ones, which, with the finer scallops, constitute the 

 gems of all early collections. The common form is Trochus 

 cinerarius, and can always be found in the living condition 

 on the rocks; the large is T. zizypliinus, and is an inhabitant 

 of deeper water, sometimes tossed on shore after storms. 

 There are, however, especially in the South, very many other 

 species, some of which live in deep water, and others on 

 the tidal rocks. Choose the species which is most abundant 

 in the locality at your disposal it will probably be Trochus 

 cinerarius and study it in the 

 living condition. Notice the small 

 conical shell, with its rounded, un- 

 notched aperture the animal has 

 no siphon, and is vegetarian. As it 

 protrudes itself from the shell, notice 

 the operculum borne on the foot, the 

 foot itself, narrow in proportion to 

 its length, the large head prolonged 

 into a non-retractile snout, and bear- 

 ing two long tentacles, and two eyes 

 placed on short stalks, which spring 



from the base of the tentacles. Between the tentacles are 

 two distinct " head-lobes," while to their outer sides lie two 

 greatly developed " side-lobes," with long, delicate processes, 

 or cirri (see Fig. 69). The cirri move about as the animal 

 progresses, and add considerably to its appearance. The 

 operculum is peculiar in being spirally coiled, as it is in all 

 the Trochi. 



As to the special characters of the shells of the different 

 species, in T. cinerarius the whorls are somewhat flat, and 

 six in number, the base of the shell shows a narrow hole 

 called .the umbilicus, the shell is marked by coarse, spiral 

 lines, and is of a dull ash colour, marked by oblique lines 



