500 MOLLUSCA 



Ocean, south to New York; it swims about near the surface in immense 

 schools, sometimes coloring the sea for miles, and forms an important 

 source of food of whales. 



SUBORDER 2. NUDIBRANCHIATA. 



Sea slugs. Shell, ctenidium, and osphradium absent, but a coiled 

 shell is present in the embryo; respiration either through the integu- 

 ment, no gills being present, or by means of adaptive gills; these sur- 

 round the anus when it is median and dorsal in position, or form longi- 

 tudinal or transverse rows; in the latter case they are called cerata 

 and usually contain hollow extensions of the liver and also nematocysts, 

 which, however, are derived from ingested hydroids; 2 pairs of tentacles 

 present : about 1,000 species, grouped in 4 divisions and 19 families. 



Key to the divisions of Nudibranchiata: 



G! Cerata usually present ; anus lateral. 



&! Cerata branched or foliaceous, in 2 rows 1. TRITONIOIDEA 



6 2 Cerata simple or absent. 



Cj Jaws present ; several rows of cerata on each side 3. ^BOLIDIODEA 



c 2 Jaws absent ; cerata usually absent 4. ELYSIOIDEA 



a, Anus median and posterior, surrounded by a circle of adaptive gills. 



2. DOBIDIOIDEA 



DIVISION 1. TRITONIOIDEA. 



Two longitudinal rows of branched cerata usually present, into 

 which the liver usually does not extend, and without nematocysts; anus 

 on right side: 7 families. 



Key to the families of Tritonioidea here described : 



a t Two pairs of cerata 1. SCYLL^EIDAE 



a. More than 2 pairs of cerata 2. DENDRONOTIDAE 



FAMILY 1. SCYLL^EIDAE. 



Body elongate, with a narrow foot; anterior tentacles absent; 2 

 pairs of branched, foliaceous cerata; male and female openings con- 

 tiguous: 1 genus. 



SCYLLJEA L. Body narrow and compressed; oral tentacles want- 

 ing; rhinophores retractile: 8 species. 



S. pelagica L. (8. edwardsi Verrill). Cerata spatula te, bearing small 

 tufted projections on the inner surface, similar projections being on 

 the back; foot very narrow; color brownish or orange, irregularly 

 spotted; length 75 mm.; width 12 mm.: pelagic, living on floating 

 seaweed. 



FAMILY 2. DENDRONOTIDAE. 



Body elongate; tentacles laminated and branched, forming fringed 

 frontal appendages; 2 rows of branched cerata on the back into which 

 the liver extends: 4 genera. 



