602 MOLLUSCA 



CirSPIDARlA Nardo. Shell slender behind and broader in front, the 

 right valve being the smaller; umbo strengthened internally by a rib: 

 numerous species. 



C. pellucida Stimpson. Shell thin, white, 12 mm. long, 7.5 wide; 

 umbo a little in front of the center: Cape Cod and northwards, in deep 

 water. 



Class 5. CEPHALOPODA.* 



Squids, devilfish, etc. Marine mollusks often of large size and of 

 high organization, which have a large head and large eyes and a circle of 

 long arms or tentacles around the mouth, and are mostly without an 

 external shell (Fig. 740, A). 



External Structure.— The body is a bilaterally symmetrical, more or 

 less cylindrical structure, which is elongated dorsally and made up of two 

 divisions, the head and the trunk. The head is the ventral portion of the 

 body, in the middle of the lower surface of which is the mouth, sur- 

 rounded by the arms. In Nautilus the arms are about ninety in number 

 and are without suckers; in the other cephalopods they are either eight 

 or ten in number and are provided with either prehensile suckers or hooks, 

 or with both these structures. 



The trunk is made up of the visceral mass (Fig. 740, A, 1) and the 

 mantle (A, 2). The visceral mass is a compact cylindrical structure with 

 delicate body walls. The mantle is thick and muscular and covers the 

 visceral mass on all sides, its lower edge being called the collar. The mantle 

 cavity (A, 4) is voluminous, and is posterior in position and contains the 

 gills and the external openings of the digestive, excretory, and reprO' 

 ductive organs. It communicates with the outside in two ways, through 

 the open space around the collar and by means of a conical, muscular 

 tube called the siphon or funnel (A, 5). By the contraction of its muscular 

 walls the water in the mantle cavity is shot violently out through the 

 siphon, and the animal is propelled rapidly through the water in the oppo- 

 site direction; water is taken into the mantle cavity again through the 

 collar space. The head and the siphon can be partially retracted within 

 the mantle by means of powerful retractor muscles. The external sur- 

 face is covered with an integument in which amoeboid pigment cells are 

 present, by the alteration in size of which the color of the animal may 

 be rapidly changed. Many cephalopods are brilliantly colored. 



A shell is present in all cephalopods except in most Octopoda. In 

 Nautilus, Spirula, and Argonaut a the shell is calcareous, external,- coiled 

 in a single plane, and in Nautilus and Spirula divided into chambers by 



• See "Report of the Cephalopods of the Northeastern Coast of America," by 

 A. E. Verrill, Rep. U. S. Fish. Com. for 1879, p. 221, 1SS2. 



