MOLLUSCA. 49 



c. Enter oäela 'plicata. 



1. 1l\x& LinguatulidcB are internal parasites infesting various organs; they 

 are flattened and tapering posteriorly, and rugose transversely. In 

 Lingaatula the interior extremity is armed with a few recurved spines. 



d. Enterodela tcvnioida. 



1. Nemertid(B. The principal genus is Nemertes^ Cuvier (j9^. 74:, ßg. 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. 



Division II. Mollusca. 



The Mollusca, or Malacozoa (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 

 {pi. 76, figs. 27, 29) ; or on the posterior portion which forms the siphons, 

 as in Unio {pi. 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 suras 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 {pi. 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 (Yermes) Mollusca was used by Linna3us in 1758, in a wide 



253 



