248 AN INTRODUCTION TO THE STUDY OF FOSSILS 



is no distinct head. A mantle sur- 

 rounds the body and secretes the shell. 



There is a general symmetry of the 

 body organs resulting from a second 

 torsion of i8o° in a direction opposed 

 to that of the original torsion of other 

 gastropods ; the primitive torsion ap- 

 pears in the course of the animal's de- 

 velopment. This secondary symmetry 

 is most manifest in the nervous system. 

 The visceral connectives are not crossed 

 as they are in Busycon, and the nerve 

 elements are concentrated in the an- 

 terior region, around the oesophagus. 



The two sexes are united in one 

 individual. 



Clio is one of the Pteropoda, — a 

 group formerly differentiated among 

 the Gastropoda, but now usually broken 

 up and scattered among various divi- 

 sions of the tectibranchs, — a sub-order 

 ,,. , . , ,. of the Opisthobranchia. Thev were 



this has a universal dis- ^ • ^ 



tribution. Much enlarged, fomicrlv Segregated on the basis of their 



ant., anterior margin of ^^^^^'^ possession 



Fig. io8. — The gastropod, 

 Clio acicula, ventral view ; 



shell ; g.o., genital opening ; 

 /./., lateral fin, — the wing- 

 like lateral lobe of the foot ; 

 li., liver ; p.f., posterior fin, 

 — the median posterior lobe 

 of foot ; post., posterior end 



of a foot trans- 

 formed into two wing-like fins and of 

 their lack of a distinct head. 



I. What characters do Clio and the 

 orsreUrre^','reproduct7ve Other members of the old class Ptero- 

 gland; St., stomach. (Re- poda posscss in common which distin- 



drawn from Lankester's aui^\i them from the TCSt of the 

 Zoology after Souleyet.) g^gtropods ? 



Tentaculites (Fig. 109). Ordovician-Devonian. 



The shell is thick-walled, conical, and tapering posteriorly to 

 an acute apex. The surface of the shell is banded with raised 

 parallel rings. 



