290 Mollusca. 



Upon the head of the Gastropoda and Cephalopoda there is 

 a pair of eyes^ which usually conform to one of the types figured 

 in Pig. 20j 2, 4—6 (p. 21) ; occasionally they are also present on 

 other parts of the body. In Chitons and LamellibranchSj cephalic 

 eyes are wanting ; if any are present in these animals they occur on 

 other parts of the body. The Mollusca possess a pair of auditory 

 vesicles with one or more calcareous otoliths [cf. Fig. 19^ p. 20) ; 

 the auditory vesicles are situated near the pedal ganglia, but the 

 nerves which supply them (auditory nerves) proceed direct from the 

 cerebral ganglia. The tentacles which are universally present in 

 Gastropods are to be regarded as tactile organs; in other 

 Mollusca, other appendages, papillae, etc., have a like function. One 

 or a pair of sensory organs (specialised portions of the skin) regarded 

 as olfactory,- occurs in the mantle cavity of most Gastropods and 

 various Lamellibranohs ; behind the eyes in the Cephalopoda there is 

 usually a pair of pits with a similar significance. 



Reproductive organs. In some Mollusca there is a pair 

 of gonads, each with its duct; but frequently there is only a 

 single gland and duct, or the glands are fused, the ducts, separate 

 and so on. In other respects, also, the reproductive system offers 

 great diversity ; many forms are hermaphrodite, others are bisexual ; 

 the ducts are often furnished with accessory apparatus; usually 

 peculiar copulatory organs of various kinds are present {see the 

 various classes) . Parthenogenesis is unknown within this group, as 

 is also asexual reproduction. 



The majority of the Mollusca undergo a metamorphosis; 

 the larva swims by means of a velum, a discoid expansion of the 

 head with a ciliated margin ; often, however, it is represented 

 only by a crown of cilia upon the head [cf., the larva of the 

 Chsetopoda) . 



The Mollusca are pre-eminently aquatic, chiefly marine 

 animals : many Gastropoda, however, live on land, but for the most 

 part in damp places. They are not only very abundant at the 

 present day, but in earlier periods were represented by a great 

 diversity of forms, and their shells are among the most numerous of 

 fossils. 



Class 1. PlaCOphora (CMtons). 



The Chitons, which were formerly incorrectly placed in the next 

 class, are a small group of Mollusca possessing a well-marked 

 bilateral symmetry almost throughout ; there is here no trace 

 of the torsion so characteristic of Gastropods. They are rather 

 flattened, somewhat convex, oval animals, with a large discoid foot 

 ventrally. Dorsally there are eight transverse shell-plates, which 

 are broad, calcareous and imbricate ; these, like the smaller calcareous 



