THE SEA-SLUGS. 255 



more active habits and graceful shape make it preferable to 

 most of the species of Doris. 



The remaining Nudibranchs. are nearly all beautiful, both 

 in form and colour, but are so numerous that we can select 

 only those which are fairly common between tide-marks. 

 Unfortunately, none of them have common English names. 



The first genus, Triopa, generally resembles Doris, but 

 differs from it in the reduction of the gills, now only three 

 in number, and the presence of slender outgrowths or 

 processes at the sides of the back. In Triopa daviger, our 

 only British species, the body is less than an inch long, and 

 is white, variegated with bright yellow, a combination of 

 colours which is very common among littoral Nudibranchs. 

 It is an inhabitant of deep water, and is only rarely found 

 between tide-marks. 



FIG. 73.Goniodoris nodosa. After Alder and Hancock. 



Another form, Polycera quadrilineata, is not uncommon 

 near low-tide mark, and is singularly beautiful in appearance. 

 It is pure translucent white, beautifully marked and spotted 

 with, bright yellow and black, the yellow spots being 

 arranged in four lines running down the sides of the body. 

 The tentacles are non-retractile, and the head bears, in 

 addition to them, four to six processes, white tipped with 

 yellow in colour. There are seven to nine simply pinnate 

 gills, and close to the gills at either side a single golden- 

 tipped process. It is to these processes with their beautiful 

 colouring that the animal owes half its beauty. I have 

 found it not infrequently among zoophytes and corallines at 

 low spring tides. It grows to a length of about an inch. 



A very similar but much smaller form is Ancula criatata 

 (see Fig. 74), which is common between tide-marks in the 



