61 



edge numerous glands secreting the purple fluid. Posteriorly the 

 mantle spreads backward in a folded tongue or lobe, the excurrent 

 siphon ; at the base of this opens the anus, either as a pore or a 

 short tube. Under the right side of mantle lies the gill, a single 

 lunate plume. In pi. 35, fig. 32, the margin of the mantle is shown 

 by the dotted line m m. Under its anterior right edge is seen the 

 genital orifice, continued in the genital groove, gr. ; behind this at 

 o is seen the orifice of the opaline gland. 



The Opaline Gland (variously known as the " grape bunch-shaped 

 gland," " gland of Bohadsch,'* etc.) is a rather large body, sometimes 

 consisting of numerous oval unicellular glands each with its inde- 

 pendent efferent duct (pi. 33, fig. 25), but usually composed of a 

 a grape bunch-like mass of cells communicating with a common 

 cavity, opening externally by one orifice (pi. 33, fig. 24). Three 

 sorts of cells compose it : odoriferous cells, color-secreting or pur- 

 purigene cells, and giant mucus cells ; the first two present the 

 same histological features, the protoplasm being granulose, the nu- 

 clei generally visible ; in the mucus glands the protoplasm is homo- 

 genous, nucleus not always visible. The gland secretes three 

 liquids : a white and odorous fluid which imparts to Aplysia its dis- 

 gusting smell, a violet and a mucous substance. In some species 

 the violet secretion is wanting. Morphologically the gland is 

 similar to the purple-secreting glands of the mantle. It is ecto- 

 dermal in origin, innervated from the pedal ganglion, and its special 

 function is apparently the secretion of odorous fluid for defensive 

 purposes., It seems to be special to the Aplysiidce, and probably 

 has no homologue in the Cephalaspidea. 



The radula in Aplysiidaa is broad, somewhat lance-head shaped 

 (pi. 33, fig. 23 ; pi. 9, fig. 13, 14), composed of many rows of nu- 

 merous, nearly similar teeth with denticulate cusps, the rachidian 

 tooth being wider, with bilobed spreading base. 



In Tethys the teeth have long cusps, closely serrate on both outer 

 and inner sides (pi. 9, figs. 11, 12, T. pundata). 



In Dolabella the radula is extremely peculiar, the teeth being all 

 unicuspid, very narrow, not serrate. See under sub-family Dola- 

 bellince. 



In Dolabrifera the denticles on the cusps are few, laterals 

 mostly tridenticulate, with no denticles on the inner margins of 

 cusps. 



In Petalifera the radula is considerably like that of Dolabrifera 

 (pi. 55, fig. 12, P. virescens). 



