XIII] CLASSIFICATION 323 



Modern cuttle-fish have a fairly uniform structure. The two 

 long tentacular arms are absent in the Octopoda. In Loligo the 

 shell is horny, in Polypus it is entirely absent. In Ommatostrephes, 

 the common cuttle-fish of the Gulf of St Lawrence, the anterior 

 chamber of the eye is open and the lens bathed with sea-water. 



Nautilus is a remarkably interesting cuttle-fish, widely different 

 from Sepia and the others, but closely allied to most 

 of the extinct forms. The arms are short, broad, ill- 

 defined lobes, the suckers being represented by tentacles with raised 

 ridges round their bases. There is a large external shell coiled forwards 

 in the median plane over the animal's head, so to speak (Fig. 149). 

 The visceral hump is enclosed in the last chamber and from the apex 

 of the hump a membranous tube called the siphuncle is given off, 

 which runs through all the other chambers, piercing the septa. 

 There is a fold of the mantle turned back over the anterior edge 

 of the shell, the first foreshadowing of the shell-sac of Sepia. 



There are four gills and four kidneys and four auricles in the 

 heart. Thus Nautilus shows traces of segmentation. The papillae of 

 the posterior kidneys are split, one half leading directly to the reno- 

 pericardial canal, the other into the sac-like kidney. The reno- 

 pericardial canals thus open directly to the exterior, and the genital 

 ducts are in such a relation to the anterior kidneys as to make it 

 probable that they are the reno-pericardial canals belonging to these, 

 which have acquired independent communication with the exterior. 



Phylum MOLLUSCA. 

 Mollusca are classified as follows : 



Class I. GASTROPODA. 



Mollusca with a flat foot adopted for crawling. There is a buccal 

 mass and radula ; distinct pleural ganglia are present and the shell 

 is never composed of paired pieces. 



Sub-class I. ISOPLEURA. 



Bilaterally symmetrical Gastropoda with a shell composed of eight 

 median plates situated in a longitudinal series. Numerous pairs of 

 ctenidia. 



Ex. Chiton. 



Sub-class II. ANISOPLEURA. 



Asymmetrical Gastropoda, with the left side of the visceral hump 

 long in comparison to the right, the anus, kidneys and ctenidia 

 being shifted forwards. 



Division I. STREPTONEURA. 



The anus and ctenidia shifted so far forward that the visceral loop 

 is pulled into the shape of an eight and the gill is anterior to the heart. 



212 



