Introduction to Animal Morphology. 275 



scale-like shell forms as a partial bivalve investment ; 

 the three-lobed foot then forms, the shell valves unite, 

 become tubular and elongated anteriorly. 



The one family, Dentaliidae, includes ten genera, 

 of which Dentalium and Siphonodentalium have an 

 entire-margined hinder opening. Antalis has the 

 hinder opening with a deep side notch. Dentalium 

 has a short, thick foot ; in Siphonodentalium it is long 

 and cylindrical. 



Sub-class 2. Pteropoda (Ctivicr) small, active, 

 marine, crepuscular, carnivorous forms, with rudi- 

 mental head and sense-organs, and large epipodia, 

 symmetrically developed as "fins," one at each side 

 of the anterior end. These are composed of two or 

 more layers of muscular fibres in a firm connective 

 base, covered by tesselated epithelium, sometimes 

 with rows of cilia (Styliola). By their rapid to and 

 fro wavings, these fins act as swimming organs ; 

 rarely a second pair of linear fins lies behind the front 

 pair (Cymodocea). The rest of the foot is rudimental, 

 with separate elements. The propodium in Pneumo- 

 dermon, Clio, &c., is elongated into retractile, lateral, 

 circumoral processes, like the arms of Cephalopoda, 

 armed with stalked or sessile (Spongobranchia) suck- 

 The mesopodium is often horsesh<>< 1 - 

 1, or both those parts maybe absent (Cleodora). 

 Tl:- odium in Limarinid;r bears a i'\v-\vhorlt'd, 



rculum. 



:ionc, rudinuMital, ov di-vrlnprd. 



Etore, and this, \\ith (ho mantle, 

 1 in tin- adult (C'ymlmliidiL'). 



vrnlrally (Cl( 18 in 



dorsally 



ward behind thr epipodia. Tho shell may be ofpure 



T 2 



