MOLLUSCS. gQj^ 



aquarium, when disturbed, its whole body is instantly illuminated by points and dots 

 of light. 



The DoEiDOPSiD^ 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 odontophore and jaw, thus presenting a marked exception to all other 

 aasteroDods. 



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

 tacles are laramate and retractile within sheaths, the shape of which varies according 

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

 o± the world. The branchia 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 



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

 forms previously described. In OncUdoris the lower pair of tentacles are replaced by 





Fig. 342. — Z)(wi5 pilosa. 



a broad membrane. In Doris the oral tentacles are distinct, and tlie branchia;, the 

 character of which is well shown in our illustrations, are capable of being retracted 

 into a cavity. Our species which are somewhat numerous, appear in favorable locali- 

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

 the 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-Oedee II. — Tectibeanchiata. 

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

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

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

 shell, which is usually present, is thin and delicate, and is not unfreqiiently 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 e^igs 

 are laid in long ribbons. "^ 



^ The first family we have to mention is the Toenatellid^, which possesses an ovoid 

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

 body is large, but usually can be completely retracted into the shell. The cephalic 

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



