POLYCERA. 



margin of the rudimentary cloak is more or less dilated, bearing on the margin tubercles, 

 dictations, or tentacular filaments. A ridge of tubercles, sometimes connected by a slight 

 membrane, extends along each side of the back, from the margin of the veil to the branchiae, 

 terminating in one or more simple or lobed branchial appendages on each side. The ten- 

 tacles, two in number, are dorsal, placed in the usual position ; they are subclavate, laminated 

 on the upper portion, and non-retractile ; without sheaths or basal filaments. The branchiae 

 are plumose, and surround the vent about two thirds along the back, whence a central ridge 

 extends to the tail. The foot is nearly linear. The aperture of the generative organs is simple, 

 and placed on the right side. 



The genus may be divided into two sections as follows : — 



Section 1. Veil dilated into tentacular filaments; branchial appendages linear, one on 



each side. Type. P. quadrilineata. 

 Section 2. Veil short, bilobed and tuberculated : branchial appendages tubercular, more 



than one on each side. Type. P. Lessonii. 



Polycera quadrilineata is generally found on Fuci, P. Lessonii on flexible zoophytes : the 

 former is therefore probably herbivorous, the latter carnivorous. 



There is not much diversity in the anatomy of the three British species of Polycera, 

 though, in some respects, P. quadrilineata differs in its organisation from its congeners. The 

 channel of the mouth is exceedingly short, leading almost immediately from the external 

 opening, situated in the inferior surface of the head, into a rather large and powerful buccal 

 organ, provided with corneous jaws, which' (PL 17, fig. 113) in the species of the second 

 section are small, and of a sub-triangular form, with the cutting edge (d) feebly developed. 

 In P. quadrilineata, they (fig. 10) are larger, and are pretty efficient cutting instruments, 

 having their upper margins concave and bearing each in front a lengthened, well-arched, 

 cutting blade (a). The tongue is moderately broad, generally of an amber colour, occasionally 

 dark : the posterior portion is tubular, and protrudes a little behind the buccal organ. In 

 P. quadrilineata there are fifteen rows of plates, with twelve plates in a row, — six on each 

 side. Two on each side, next the median line, bear bicuspid spines ; the innermost of these 

 have the spines considerably smaller than the others : the external plates are of an elongated 

 form, a little elevated in the centre, and diminish in size towards the external border. The 

 tongue is divided down the centre by a naked space, the central plate being wanting. In 

 the other two species, the lingual organ is similar to the above, but in them the spines are 

 shorter and stouter, and the innermost are proportionally smaller. In P. ocellata, the rows of 

 plates are sixteen in number, with seven in a row on each side : in P. Lessonii there are only 

 thirteen rows, eight plates on each side, and the naked space down the centre of the tongue is 

 of considerable width. 



The oesophagus (fig, 9c) is a rather long, simple tube, and has at its origin on either side 

 a small salivary gland, (b) which opens into the buccal organ {a) : these glands are tubular and 

 are more or less folliculated. The oesophagus, passing backwards, enters the anterior border 

 of the liver on its way to the stomach, which is small, and is buried in that viscus, the upper 

 wall (d) being visible at the surface above and towards the left side. Here the intestine (e) 

 leaves the gastric pouch, and passing across the liver to the right, arrives at the anal termina- 

 tion (/) in the midst of the branchial circle. The intestinal tube is rather short, and in 



