Sea- slugs 2 8 i 



usually no jaws. The most noticeable of this section 

 are the Sea Lemons, so-called from the resemblance 

 of one species to the half of a lemon ; at least this 

 was the likeness that struck the naturalists early 

 last century. To-day the marine zoologist admires the 

 remarkable mimicry of sponges in their colours and 

 markings. They are elliptical in outline, and more or 

 less flattened. They arc completely covered above by 

 the mantle, which extends even beyond the head and 

 foot, and is seldom smooth, being usually thrown up 

 into little tubercles or granules and stiffened by having 

 calcareous spicules embedded in its substance. The 

 eyes are mostly hidden by the integument, and can 

 only be seen by dissection or in the larval stage. 

 The pair of tentacles on the back come through 

 apertures in the mantle, and are covered with over- 

 lapping plates; these are the sense-organs {vJiino- 

 Ijhores). At the other end of the Sea Lemon's back 

 is a rosette formed by spreading leaf - like organs 

 arranged round a central cavit}^ This is the opening 

 of the anus, and the surrounding leaves are the gills, 

 all connected at their base and capable of being 

 retracted into a cavity in some species, though not 

 in all. 



The best-known member of this group, and that to 

 whicli the name Sea Lemon was first applied, is the 

 Rough Sea Lemon (Doris tuherculata), which is 

 usually about 3 inches in length, though examples 

 may sometimes be found as long as 5 inches. The 

 prevailing colour is pale yellow or dull orange, but it 

 varies from white to orange, and may be marbled or 

 clouded with grey, pink, and green. The encrusting 

 sponges of the genus HalicJiondria, especially Hall- 



