MOLLUSCA. 49 
c. Enterodela plicata. 
1. The Linguatulide are internal parasites infesting various organs; they 
are flattened and tapering posteriorly, and rugose transversely. In 
Linguatula the interior extremity is armed with a few recurved spines. 
d. Enterodela taenioida. 
1. Nemertide. The principal genus is Vemertes, Cuvier (pl. 74, fig. 10), 
an extremely long and soft marine worm, of a flattened form, the anterior 
extremity slender, and the posterior one ending with a broad attaching disk. 
It approaches Hirudo in its mouth and vascular system. It lives buried in 
‘the sand, and is said to feed on shell-fish. 
Drviston Il. Mottuvsca. 
The Mottusca, or Matacozoa (soft animals), constitute one of the great 
Divisions of the animal kingdom. Except the shell, which is not always 
present, these animals have nothing in the nature of bones; and they want 
the ringed structure and jointed members of the Articulata. The body 
is soft, the integument slimy, and generally without epidermis, and in the 
testaceous species capable of secreting a shell. The alimentary canal has 
an opening at each end, and the circulating system is more or less complete. 
The eyes of univalve mollusca are generally situated upon or near the 
tentacles; those of bivalves upon the edge of the mantle, as in Pecten 
( pl. 76, figs. 27, 29); or on the posterior portion which forms the siphons, 
as in Unzo (pl. 76, fig. 47). 
From the great number, variety, and beauty of the shells of so large a 
portion of the Mollusca, the study of these, under the name of Conchology, 
attracted attention at an early period, when a cabinet of shells was often 
regarded in the same light as a casket of jewels, and great sums were paid 
for rare and handsome species; and, indeed, this interest still maintains its 
ground, there being shells, the price of which is one or two hundred dollars 
at the present day. 
It was at length discovered that little of natural classification and the 
habits of this class could be known, without a study of the entire animal; 
so that Conchology finally became merged into Malacology. There is, 
however, no impropriety in the use of the former term in an enlarged sense, 
if it be considered to include the study of those animals which are usually 
provided with a calcareous shell. 
Some of the older conchologists, guided by the shell alone, included 
radiated forms, like Echinus, in this division; and even at the present-day, 
the cirrhopoda (pl. 76, figs. 51-54) and some of the annelida, which have 
a hard exterior tube, are sometimes described in books avowedly devoted 
to mollusca. On the other hand, whilst all ‘“ shells,” whether secreted by 
the mollusca or the articulata, were classed together, true mollusca, when 
unprovided with a shell, were often placed among worms and annelida. 
The term (Vermes) Mollusca was used by Linnzeus in 1758, in a wide 
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