CLASS IV. SCAPHOPODA, Bronu. 

 (= Solenoconcha, Lacaze-Duthiers). 



MARINE bilaterally symmetrical Prorhipidoglossomorpha ; the body 

 and shell elongated along the antero-postcrior axis, and nearly cylin- 

 drical. The right and left margins of the mantle are united vent- 

 rally, and thus form a complete tube surrounding the body, but with 

 an anterior and a posterior aperture. The head is somewhat rudi- 

 mentary and devoid of eyes, but bears 2 dorsal appendages furnished 

 with numerous long filaments. The foot is cylindrical, and adapted 

 to digging. A radula is present, but there is no ctenidium. The 

 sexes are separate. 



The shell has the form of a very elongated cone, slightly curved, 

 the concavity of the curve being dorsal ; it is capable of containing 

 the entire animal. The larger orifice of the shell is the anterior or 

 cephalo-pedal aperture. Near the smaller posterior aperture the 

 shell, being older, is also thicker. The posterior aperture is emar- 

 ginated by a ventral sinus, and is furnished interiorly with a dorsal 

 and a ventral valve, which are capable of being applied to one an- 

 other. The animal lives buried obliquely in the sand, only the pos- 

 terior aperture projecting into the water, and therefore it is the 

 posterior extremity that is at once inhalant and exhalant and serves 

 for the expulsion of the excrements and the genital products. 



The buccal cavity has a simple dorsal mandible and a ventral 

 radula, which is short and arcuate, with 5 teeth in each transverse 

 row. The central tooth is simple and sub quadrangular ; the laterals 

 stout and subtrigonal, tricuspidate, with wide bases and reflected 

 borders ; the marginals have the form of subquadrangular non-denti- 

 culate transverse plates. 



The structure of the circulatory system is exceedingly simple ; 

 there are no differentiated vessels, not even a ventricle with well- 

 developed muscular walls. There are 2 symmetrical kidneys, situated 

 in front of the gonad on the ventral side of the middle of the body. 

 The nervous system comprises the same 4 pairs of principal nerve- 

 ganglia as are found in the Gastropoda. The sexes are always sepa- 

 rate. The gonad is unpaired and median, and is extremely long, the 

 sexual products being emitted through the right kidney. The em- 

 bryonic shell is formed of 2 calcareous laminae, which subsequently 

 unite to form the tube. 



With regard to their general relationships, the Scaphopoda re- 

 semble the Gastropoda in their univalve shell, and in the possession of 

 a radula ; while the pointed foot, the non-lobed velum in the veliger, 

 the generative system, the bilateral symmetry of the organs generally, 



