338 THE TECTIBRANCHIATA. 



the greater number of species, the mantle is fringed with long filamentary appendages, but in 

 the present example the edges are quite plain. It is, however, always flat and much 

 expanded. The tentacles are long and slender, and the eyes placed at their bases. 



The Physas and Planorbis, Lymneas and the delicate Pond-shells are of the order PUL- 

 MONATA. The Helices are included ; quite familiar to us as Garden-snails. The common 

 Garden-snail of Europe, Helix Jiortensis, was introduced into Essex County, Mass., many 

 years since, and now is quite frequently found in the vicinity of Salem and Lynn. Helix 

 alternata is the most abundant species in Eastern New England. A small area on Bass 

 Point, Nahant, is a favorite locality, and in no other place is it found within several miles. 

 H. albolabris, or White-lipped Snail, is also Common. Numerous species of Pupas are 

 abundant in the same region. A familiar form of this group is the Naked Slug, which has 

 but a thin film of shell on its back, and is called Garden Slug (Dimax). 



OPISTHOBRANCHIATA. 



WE -now come to some of the strange and almost grotesque forms which are assumed by 

 many of the mollusks. These belong to a fresh order, in which the shell is sometimes alto- 

 gether wanting, and even when present is of very small dimensions, and is almost, if not 

 wholly, concealed by the soft parts. In fact, they may be considered as the marine analogues 

 of the common land slugs. The gills of these animals are rather curiously formed, not being 

 placed in a definite cavity, as is the case with the previous species, but projecting boldly from 

 the surface of the animal, and set towards the rear of the body. On account of this position 

 of the gills, the animals are termed Opistho-branchiae, or Rearward-gilled mollusks. The 

 whole internal structure of these creatures is fully as curious as their external form, and well 

 repays dissection, the organs of digestion especially being rather complicated, and possessing 

 many points of interest. 



The BUBBLE-SHELL, spotted on the exterior, is one of a moderately large genus of mol- 

 lusks, that are found in almost all tropical and temperate seas, and may generally be captured 

 where the bed of the ocean is of a sandy nature. In all the family to which the Bubble-shells 

 belong, the shell is very thin, globular, and cylindrical, and the aperture is long and rounded. 

 The large side lobes are said to be often used as fins for swimming. 



The APLTTSTRUM is a closely allied species, formerly placed in the same genus, but now 

 separate on account of several structural differences. It has a shell rounded and slightly 

 colored, but small in proportion to the size of the animal. The foot of the Aplustrum is 

 exceedingly large, capable of concealing the shell entirely in its folds. Behind the tentacles 

 are four large and flattened lobes, and the eyes are very small and set at the inner bases of the 

 tentacles. 



A VEEY curious creature is the Bulla Ampulla. 



This is an example of a gemis termed CYLICHNA ; it is remarkable for one or two peculiari- 

 ties. The animal is smaller in proportion than is generally the case with this family, being 

 short and broad, and not able to wrap its lobes over the shell. The head is flat, blunt in front, 

 and the eyes are deeply sunk in the tissues. The shell is cylindrical and mostly smooth, and 

 the aperture is narrow. 



A curious animal is the BOATMAN'S SHELL, a specimen of a very small genus, containing 

 only five species. It is a sand-loving creature, mostly remaining in a rather considerable 

 depth, and being found in fifty fathoms of water. This animal has no eyes ; and althoitgh the 

 side lobes are turned up, and are very large, they do not envelop the shell. 



Sub-order TECTIBRANCHIATA. In some of its families the animal has an ovoid shell, as in 

 the Bullas, or Bubble-shells; the curious Aplysias, or "Sea Pigeons," great masses of flesh, 

 with no shell, and wide flaps for swimming. These are common forms in the tropical waters. 

 Sea Hare is another name, from fancied resemblance. We have seen these creatures throw 

 out their purple ink of great density, clouding the surrounding water, evidently for the 

 purpose of defeating its enemies and securely changing its locality. 



