METAZOA MOLLUSCA. 243 



like Clio (No. 577) and moves by means of bands of cilia. 

 The adult (No. 579, No. 580, P. violaceum d'Orb.), pos- 

 sesses a distinct head which carries specialized tentacles 

 having suckers similar to those of cuttlefishes. At the 

 posterior end of the body there are external gills (No. 

 580). 



Ciionopsis krohni (No. 581; No. 582, C. flavescens 

 Gegenbaur) has a barrel-shaped, shell-less body which 

 is nearly transparent, and the visceral mass extends to 

 the posterior end. The head is small with no mouth 

 appendages. 



Section 10. CEPHALOPODA. 



The Cephalopoda and Brachiopoda are perhaps the best 

 groups that can be chosen at the present time to illustrate 

 a natural classification based upon the stages of growth 

 and decline. It is obviously impossible to follow out this 

 classification in detail, to trace the numerous branches of 

 the Cephalopod trunk to say nothing of the minute twig- 

 like divisions and subdivisions of these branches. It is 

 hoped , however, that such a series of specimens and draw- 

 ings has been selected, and such an arrangement of these 

 made, as will give the student a clear general view of the 

 subject. 



It is well to impress upon one's mind at the very out- 

 set the truth of the words: "A single shell, either from a 

 living or fossil form, may present, accurately, the general 

 history of the development of the young, the stages of the 

 adult and of old age." x 



1 Hyatt, Fossil Cephalopods in the Museum of Comparative 

 Zoology, Proc. Amer. Assoc. Adv. Sci., XXXII, 1883, p. 323. 



