MOLLUSCS. 301 



aquariuin, when disturbed, its whole body is iiistiiiitly iHuininated 1)y points and dots 

 of light. 



The DoKlDOrsiD--E is noticeable from the fact that the species, which in general 

 appearance resemble those of the next family, have a sucking mouth, and 'are desti- 

 tute of an odonto])liore and jaw, thus presenting a marked exception to all other 

 gasteropods. 



The last and largest family of the nudibranchs is the Doeidid^, in which the ten- 

 tacles are laminate and retractile within sheaths, the shape of \\hich varies according 

 to the genus. There are about four Inmdred known species distributed in all the seas 

 of the world. The branchiae vary considerably in shape, but are usually branched, 

 and when expanded, the circle presents a close resemblance to a flower, the effect of 

 which is strengthened by the brilliant colors which are frequently present. 



Sjjecies are most numerous north of Cape Cod. In their habits they resemble the 

 forms previously described. In Oncliidoria the lower jiair of tentacles are re]j]aced by 







a broad membrane. In Doris the oral tentacles are distinct, and the branchiae, the 

 cliaracter of which is well shown in our illustrations, are ca]iable of being retracted 

 into a cavity. Our species which are somewhat numerous, ap])ear in favorable locali- 

 ties in large numbers; but, owing to the protective coloration, which may be similar to 

 tiie dull sea-weeds or the bright hydroids among which they dwell, they readily escape 

 the collector's eye. Other dark-colored forms are frequent under stones at or near low- 

 water mark. 



Sub-Order II. — Tectibranchiata. 



The name for this group is the antithesis of that employed for the last, and is used 

 to indicate the fact that the gills arc covered and concealed by a flap of the mantle. 

 The gills, it should be said in jiassing, are not homologous throughout the group. The 

 shell, which is usually present, is thin and delicate, and is not unfrequently concealed 

 by a flap of the mantle which is bent back over it. Another fact of importance is the 

 great development of the epipodia found in most members of the group. The eggs 

 are laid in long ribbons. 



The first family we have to mention is the Tornatellid.e, which possesses an ovoid 

 spiral shell, which is usually marked with one or more s])iral rows of punctures. The 

 body is large, but usually can be com])letely retracted into the shell. The cephalic 

 tentacles are large and broad, and united at the base, while the eyes are situated on 



