206 SYSTEMATIC ARRANGEMENT 



Synopsis or Suborders. 



I. Pygobranchia. Gills on the middle of the hinder part of the 



back, surrounding the vent, plumose or branched. Skin spiou- 

 lose. 



II. Inferobranchia. Gills on the under side of the expanded 



mantle, lamellar. 



III. Polybranchia. Gills on the upper surface of the mantle, 

 lamellar or plumose. 



IV. Cerabranchia. Gills on the upper surface of the mantle, 

 fusiform, papillary, simple or branched. 



V. Placobranchia. Gills on the upper surface of the mantle, 



lamellar or venose, spread over the surface. 



VI. Pe[,libranchia. Gills none distinct, the function being per- 

 formed by the skin. 



Suborder I. Pygobranchia. 



Gills on the middle of the hinder part of the back, surrounding 

 the vent, plumose or branched. Skin spiculose. 



The skin of the Doridee is tough and coriaceous, and is of a spongy 

 or cellular structure within the mantle in all the British species 

 examined, stiffened with numerous imbedded spicula having a more 

 or less symmetrical arrangement. Spicula are also observed on the 

 foot, on the tentacles, and on the root of the branchial plumes. The 

 under surface of the foot, the upper surface of the cloak, the bran- 

 chial plumes, and the dorsal tentacles, are covered with vibratile 

 cilia. — Hancock fy Embleton, Phil. Trans. 1852, 243. 



Professor Ehrenberg observes, " In tota Doridinorum familia max- 

 illae desunt; in hac forma palatum denticulorum densa fascia fusca 

 armatum est, denticuli vero non seriatim dispositi sunt." — Symb. 

 Phys. Moll. i. 



A. Body convex. Mantle large, margining the foot, without mar- 

 ginal appendages. Dorsal tentacles two, retractile into cavi- 

 ties. Oral tentacles two, or wanting (p. 211). 



a. Lingual membrane narrow. Teeth in two principal longitudinal 

 series, and sometimes two smaller series (p. 208). 



Fam. I. ONCHIDOR1ILE. 



Body convex. Mantle large, margining the foot. Dorsal tenta- 

 cles laminated. Head veiled. Branchiae non-retractile. Lingual 

 membrane narrow, with two> longitudinal rows of teeth, and some- 

 times one or two lateral rows of smaller teeth on each side. 



