232 THE SEA SHORE 



symmetrical form of both body and shell and in the total or almost 

 total absence of the foot ; and this distinction is so marked that 

 the pteropods are often separated from all the other Ceplialophora 

 into a class by themselves, while all the remainder are placed in 

 a separate extensive class called the Gasteropoda, because they 

 creep on the ventral surface of the body, the term signifying 

 stomach-footed. 



These gasteropods are divided into four orders : the Nucleo- 

 branchiata, in which the respiratory and digestive organs form a 

 nucleus on the posterior part of the back ; the Opisthobranchiata, 

 with gills more or less exposed towards the rear of the body ; the 

 Pulmonifera, or lung-breathing order ; and the Prosobranchiata, 

 in which the gills are situated in advance of the heart. The third 

 order includes all the land snails and slugs, and does not therefore 

 fall within the scope of our work ; but the remaining three consist 

 either exclusively or principally of marine species, and will be 

 dealt with in the order in which they are named. 



The Nucleobranchs are not really gasteropods in the strictest 

 sense of the term, for they do not creep along by means of their 

 foot, but all swim freely in the open ocean, always at the surface, 

 and sometimes adhere to floating weed by means of a sucker. In 

 fact, the foot of these creatures is greatly modified in accordance 

 with their habits, one part being often expanded into a ventral 

 swimming fin, and provided with a sucking-disc for adhesion, 

 and another produced into a posterior fin for locomotion. 



Like the pteropods, the nucleobranchs are purely pelagic, so 

 that we can hardly expect to meet with a specimen on or near the 

 phore ; and thus we shall content ourselves with a brief notice of 

 their general characters. 



The shell is very variable in size and form, and sometimes 

 even entirely absent. Large-bodied species often possess but 

 a very small shell, while some are able to entirely retract them- 

 selves and close the mouth of the shell by an operculum. These 

 animals are generally provided with a large cylindrical proboscis, 

 and the tongue has recurved teeth. The body is usually very 

 transparent, often so much so that the blood may be seen circula- 

 ting within it, and the nervous system is much more perfectly 

 developed than in the pteropods. The eyes, tod, are perfectly 

 formed. 



The presence of special breathing organs may seem to be super- 

 fluous in such delicate and soft-bodied creatures as these, for it may 



