Introduction to Animal Morphology. ly^ 



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 [Cuvier) — 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- 

 ing discs. The mesopodium is often horseshoe- 

 shaped, or both these parts maybe absent (Cleodora). 

 The metapodium in Limacinidse bears a few-whorled, 

 spiral operculum. 



The mantle may be none, rudimental, or developed, se- 

 creting a shell on or in its texture, and this, with the mantle, 

 may be persistent or lost in the adult (Cymbuliidae). Its 

 lobes may bound a cavity opening ventrally (Cleodora), as in 

 Cephalopoda, or dorsally (Limacina) by a slit directed for- 

 ward behind the epipodia. The shell may be of pure con- 



T 2 



