50 GARSTANG : STRUCTURE AND HABITS OF JORUNNA JOHNSTONI. 



one of which runs down the middle of the back, while the other 

 two run backwards on either side from the rhinophores to the region 

 of the branchiae. Differentiations of the dorsal integument along 

 these three lines occur also in several allied genera, e.g. Goniodoris, 

 Idalia. It is the general arrangement of the dark spots in these 

 three lines which is constant ; the actual position assumed by them 

 upon the lines is subject to extensive variation in different individuals. 

 The most symmetrical condition that I have met with is one in 

 which there are two spots in each longitudinal row, the anterior spot 

 in each lateral row being midway between the rhinophore and the 

 posterior spot. Under this arrangement the brown-pigmented 

 rhinophores seem to be the anterior members of the lateral rows of 

 spots and easily escape recognition. Often there are no spots in the 

 middle line at all (see Mcintosh's figure); more rarely there are 

 more spots in the middle line than in either of the lateral lines (see 

 Alder and Hancock's fig. 2). Lastly, an asymmetrical arrangement 

 of the spots in the lateral lines is very frequent : there may. for 

 instance, be only one spot on one side, while there are two, or even 

 three, on the other. 



Another feature which is highly characterestic of this species is 

 met with in the disposition of the branchial plumes. These vary- 

 slightly in number, from 12 to 16 in all; and when expanded, as 

 is almost always the case, they form a complete circle, enclosing the 

 anus, and assuming the shape of a conical, tubular or cup-shaped 

 prominence, according as the number of plumes present is smaller 

 or greater. 



Alder and Hancock have described the back as being " closely 

 covered with very minute, equal, linear, and spiculose tubercles, 

 scarcely visible to the naked eye, and giving the cloak a granular 

 appearance," and one of their figures (figure 4) illustrates this state- 

 ment. This account is correct and distinctive so far as it goes, but it 

 is inadequate in regard to the structure of the individual tubercles, 

 and the figure can only have been drawn from a specimen in very 

 poor condition. In a fresh and healthy individual the tubercles are 

 seen to have the shape of slender inverted cones, armed around 

 their sides with 5 or 6 stout spicules, which project freely beyond the 

 upper margins of the cones. Each tubercle, moreover, terminates in a 

 slender tentacular process which possesses powers of slow contraction 

 and extension. When contracted, the tentacular process is quite 

 concealed within the crown of spicules ; but when fully extended it 

 protrudes for a considerable distance beyond them. Its surface is 

 covered with cilia, and these appear to be stiffer and less vibratile at 

 the apex than on the general surface. Near the tip of each tentacular 



