AMPHINEURA. 



345 



body wall (ctenidia or gills), but one or both of these may 

 be absent. I At the base of the gills there is generally an olfac- 

 tory organ or osphradium. ) Frequently there are two larval 

 stages, the Trocho sphere, which resembles the same stage in 

 some Annelids, and the more characteristic Veliger ; but the 

 development is often direct. 



Class I. AMPHINEURA. 

 Syn. GASTEROPODA ISOPLEURA, e.g., Chiton. 



GENERAL CHARACTERS. The Amphineura are marine 

 Molluscs, more or less elongated in form, with bilateral 

 symmetry. They are often ranked along with Gasteropods. 



The mouth is anterior, the anal 

 and nephridial apertures are pos- 

 terior. The mantle, which bears 

 cuticular spicules, covers at least 

 a great part of the body. I The 

 nervous system consists of a cere- 

 bral commissure and two paired 

 longitudinal cords, with ganglionic 

 cells, but at most very feeble ganglia, 

 which run the whole length of the 

 body. Of these paired cords the 

 pedals are connected by numerous 

 cross commissures, and the vis- 

 cerals or pallials are united pos- 

 teriorly by a commissure above the 

 rectum. The bilateral symmetry is shown internally, e.g., in 

 the paired nephridia, auricles, and genital ducts. The class is 

 of ancient origin, dating from the Silurian. There are two 

 orders : Polyplacophora, e.g.. Chiton, and Aplacophora, 

 e.g., Neomenia. 



ist Order of AMPHINEURA, POLYPLACOPHORA (Chitonicke). 



The members of this order, represented on British coasts by several 

 species of Chiton, are sluggish, usually vegetarian, animals, occurring 

 from the shore to great depths. The foot is generally as long as the 

 body; the mantle covers the back and bears eight shell plates (Fig. 112), 

 perforated, in many cases at least, by numerous sensory organs, which 

 may be in part optic ; numerous gills lie in a regular row along a groove 

 on each side between the mantle and the foot. 



FIG. 112. Chiton. 

 / PRE"TRE. ) 



(After 



