Class 3. Acephala. 



313 



testis are present as paired branched organs ramifying between the 

 other viscera, in the foot, or {e.g., in the Mussel) in the mantle; 

 they open on either side close to the nephridial aperture, or the 



■m 



Fig. 258. Diagrammatic longitudinal section of a LameUibranch. a 

 anus, br gill, <. cerebro-pleural ganglion, e rectum, / foot, fg pedal ganglion, g generative 

 gland, g' genital aperture, h ventricle, i visceral ganglion, h CEecum containing the orystaUine 

 stylet, I liver, m anterior, m' posterior adductor, ma stomach, n kidney, n' external renal 

 aperture, o mouth, p mantle, pd pericardial gland, pe pericardium, v auricle. — Adapted 

 from Eankin. 



duct unites with that of the kidney, so that a common urino-genital 

 aperture is present on each side.* Sometimes the fertilised eggs 

 develop in the space between the lamellse of the outer gills of the 

 female. 



Marine Lamellibranchs undergo 

 a metamorphosis similar to 

 that of the marine Gastropods ; 

 the newly -hatched larva moves by 

 means of a velum; eyes are 

 frequently present anteriorly, but 

 disappear later. Such a free-swim- 

 ming larva does not occur in most 

 fresh-water forms, and a velum is 

 not developed or is only transitorily 

 present in the embryo. 



All Lamellibranchs are aquatic, 

 the majority marine. They 



Fig. 2.59. Larvae of a Cookie 

 {Carrliiim) ; in the older a small shell s 

 is developed, v velum. — After Lovun. 



* In some the reproduofclTe organ opens into the kidney itself, in certain oases even 

 close to the reno-perioardial aperture. 



