490 



MOLLUSCA 



tudinal ridges ; up to 9 cm. long and 55 mm. wide : West Indies, Bermuda, 

 Florida, Texas; the commonest West Indian Chiton. 



C. squamosus L. (Fig. 745). Shell greenish in color and with median 

 keel; central areas of shell longitudinally, lateral areas transversely 

 striated; length 8 cm.: Bermuda and West Indies; Europe. 



CLASS 2. SCAPHOPODA.* 



Bilaterally symmetrical, elongated mollusks enclosed in a tapering 

 cylindrical shell, which is open at the two ends and slightly arched, the 

 concave side representing the upper or dorsal side of the body (Fig. 753). 

 The mantle folds arise in the larva from the dorsal 

 surface of the very much elongated visceral mass and 

 fuse together ventrally, forming a cylinder open at 

 both ends. From the anterior and larger opening 

 projects the cylindrical foot (7) and a number of slen- 

 der tentacles (8). The mantle cavity is posterior in 

 position. The head is not developed, the mouth being 

 situated at the extremity of a cylindrical snout (5), 

 where it is usually surrounded by small leaf-like ten- 

 tacles, while from the base of the snout extend the 

 long slender tentacles above mentioned. No ctenidia 

 are present, the tentacles being possibly homologous 

 to them. 



The mouth opens into a V-shaped intestine, in the 

 anterior portion of which is a jaw and a radula; the 

 anus is in the mantle cavity at the base of the foot. 

 The heart is rudimentary, no auricles being present. 

 The nervous system is well developed, paired cerebral, 

 visceral, pedal, pleural, and buccal ganglia being pres- 

 ent. No organs of special sense except lithocysts are 

 found. A pair of nephridia opens to the outside by 

 two pores near the anus. The sexes are separate, the single gonad dis- 

 charging its products through the right nephridium. 



History. The older authors called the scaphopods Tubulus marinus 

 or Denticulus elephantis, and placed them among the tubicolous annelids 

 with Serpula, or among the snails, in the neighborhood of Patella or of 

 Vermetus. In 1857 Lacaze-Duthiers placed the group under the Acephala, 

 maintaining that the twofold origin of the mantle showed a nearer rela- 

 tionship to bivalves than to snails. M. Sars and Bronn, however, 



* See "Scaphopoda," by II. Simroth, Bronn's Klassen u. Ord. d. Thier, Vol. 3, 

 1 Abt, 356, 1894. "Scaphopoda," by H. A. Pilsbry and B. Sharp, Man. of Conch., 

 Vol. 17, 1897. 



Fig. 753 Dia- 

 gram of Dentalium 

 (Lang). 1, gonad ; 

 2, shell ; 3, retrac- 

 tor muscle; 4, 

 anus ; 5, mouth ; 



6, mantle cavity ; 



7, foot ; 8, cephalic 

 tentacles. 



