44 MOLLUSCA. 



Sub-order 1. Placophora.* Body vermiform, symmetrical, with- 

 out eyes and tentacles. Ventral surface flattened ; dorsal surface 

 covered by calcareous plates placed in a segmental manner one behind 

 the other. Gills and kidney paired. 



The Placophora are the most nearly allied of all Mollusca to certain 

 forms of worms, to which they approximate through the genera 

 Neomenia and Chceioderma. The symmetrical body does not possess 

 a separated head, eyes, or tentacles. The integument presents 

 numerous scattered spines, which are sometimes hard and chitinous, 

 and sometimes calcified ; they always arise in special follicles lined by 

 ectoderm cells. In addition to these integumentary structures, which 

 are also present in Chcetoderma, there are a series of broad calcareous 



plates on the dorsal surface, which are only 

 exceptionally covered by the mantle (Crypto- 

 chiton), and which, according to their origin, 

 represent a multivalve Molluscan shell. The 

 free edge of the mantle is moderately thickened, 

 and under it on each side is placed the small 

 mantle cavity as a furrow containing a series 

 of leaf-like gills (fig. 519). 



Of special interest is the simple condition of 

 the nervous system (fig. 495), which greatly 

 resembles that of the Gephyrean-like genera 

 Neomenia and Clicetoderma. Cerebral ganglionic 

 FiG.5i9.-c*J H (tpiniferus) swellings are absent, in correspondence with 



,pino*H* (regne animal). ^ ^^ Q eyeg an(J tentacleg> Four nerve 



trunks pass off from the double oesophageal ring, an upper lateral 

 pair, the pallial nerves, and a ventral pair, the pedal nerves, which 

 latter are connected by transverse commissures. Pedal and visceral 

 ganglia are not separated as ganglionic swellings from the nerve 

 stems. Buccal ganglia, on the contrary, are present. 



The alimentary canal begins with the mouth, which is placed on 

 a roundish lobe ; it is much coiled, and extends through the whole 

 length of the body, to open by the anus at the hind end. As in 



* A. Th. Middendorff, " Beitrage zu einer Mnlaoozoologica rossica. 1. 

 Beschreibung und Anatomie neuer oder fur Kussland neuer Clntonen." Mem. 

 Acad. Jii/j).,St. Pcterxlnrg, 1848. 



S. Loven, " Ueber die Entwickelung der Gattung Chiton." ArcJiiv fur 

 Natwrgescli., 3856. 



B. Haller, " Die Organisation der Chitonen der Adria." Arleiten a. d. Ziwl 

 lust, in Wicn., Tom IV., 1882. 



Vide also Tullberg's and Graff's works on Neomenia and Cli(etoderina. 



