25-3 ZOOLOGY. 



then deTeloped. The foot gradually lengthens and forms the 

 characteristic fin or keel. The Telum is meanwhile ab- 

 sorbed, the operculum (Carinan'a), or the operculum and 

 shell, are thrown off, and the larva gradually assumes th& 

 form and organization of the adult. The close relationship 

 of the Heteropods and Gastiopods is shown by the great 

 similarity of their larvae. Gegenbaur even goes so far a? to 

 include them under the Opixtlinhraiichiala, while von Jher- 

 ing unites them with Chitons and some other forms under 

 the name ot ArfJirocorJilidce ; for the present it seems best 

 to retain them as a subclass. 



The fossil genus BeJlerojjhon is closely related to the 

 Atlaniidm. 



Class II.— CEPHALOPHORA. 



Molhiiiks i.rith a distinct head, itUJi terdades, eye» and enm in the JicA ^ 

 the foot f'^rrniug a creeping disk; ihebody eii/ier naked arulbiUUeraUy ivra- 

 met'ric'd, or enclosed in tt /tpirnZ KlifM, and coriAeq'icuUyhekind the hiai 

 asymmetricil. Mouth with pharyn^jeM ieeUi and a Ungual rUbon (odon- 

 tophore). Xervons syUem ctjngUiting of four pairs of gahylin, the brain. 

 teeU decdopt:d. The inte-^tlae usually ending nenr the m^j'/jji. Xlii heart 

 with usually a. single auricle. Breathing by a Hngle gill, or a lung like 

 giU, ; a double Mdruy.but forming a single ;«<!>-!. S xes unUed or sepa- 

 rate. Young pn.H.<ing through a morula, gaslruln, sumelimes a troelio- 

 gphere and usually a zeliger stage; in the land-snails deteWpmtnt 

 direct. 



B'j/jda--s 1. Scaph-ypoda. — Xo bead, several long- thread-like tentacles ; 

 foot lonjr, trilobed. i^\vW long, conical, open at each end. 

 A single order SAenOconchos. (Dentalium, Siplionodenta- 

 lium.) 



S'Adasi'i. Pteropoda — Body with two wing-like espansyn* ^velum) 

 on the front part of the foot, for ^wimmlujr : bo<ly naked or 

 shelled. HennapbrMditic. Larva with a velum and shell. 

 Order 1. r/i'.'_-o«/(WSrtto(Hyalea, Cleodora, Cavolina). Order 

 2. Gymnosomata. (Clione). 



S'lhclasiZ. OiiMropoda. — Order 1. P i oiohraiichiata (Halintis, Patella, 

 Trocbns, Littorina, Lonatia, Pa'adina, PurriteUa, lanihica. 

 Cypraea, Strombns, Cassis, Buccinuin, Xassa, Purpura.) 

 Order -2. Opisth/ibraiichiata. (Bulla, Aplysia. Eolis, Doris.) 

 Orders. Pulmonata. iLimnaeus, Planorbis.Auricola, Helix. 

 Bulimas, Liuiax). 



