MULLERIA. 199 



other, to the mantle and to the body-wall, are those of a typical 

 Eulamellibranch. The same is true of their more intimate 

 structure, for an examination of sections under the microscope 

 reveals the fact that the two halves of each gill lamella are 

 connected together by vascular mterlamellar junctions, and that 

 the gill filaments themselves are further similarly united. In the 

 individual filaments we find a supporting tissue, and firmer sup- 

 porting rods are present. The whole structure recalls in its most, 

 minute details that of the gills of Anodonta or Unio" 



" The heart consists of a muscular ventricle and two thin- walled 

 auricles, the former giving origin to an anterior and a posterior 

 aorta, the whole being enclosed in a fairly spacious pericardium. 

 A mass of brown tissue forms the side-walls of the pericardium 

 anteriorly ; this evidently represents Keber's organ." 



" The most striking feature seen in connection with the heart 

 is the fact that the ventricle is not perforated by the rectum, but 

 is situated some little distance below the latter, being separated 

 from it by a portion of the genital gland. The pericardium, 

 moreover, does not surround the rectum as in Anodonta, its roof 

 being formed by the thick mass of the genital gland, within which 

 the rectum lies embedded." . 



' ; The Kidney or Organ of Bojanus. The kidney is 



essentially that of a typical Eulamellibranch, and closely resembles 

 that of Anodonta in every detail." 



" The Genital Organs : A large genital duct opens close to, 

 but perfectly independent of, the external opening of the kidney 

 in the super-branchial chamber. The genital gland is extensive ; 

 it surrounds the coils of the intestine, and even extends round the 

 rectum above the pericardium." 



" The Alimentary Canal : The mouth practically leads direct 

 into the stomach, an oesophagus, as such, not being present. The 

 digestive glands are situated below and at the sides of the stomach, 

 into which they opsn. There is no crystalline style or " fleche 

 tricuspid." The intestine is fairly long, and bent on itself several 

 times ; before leaving the visceral mass it enlarges abruptly ; this 

 point evidently represents the junction of the mid-gut with the 

 hind -gut. The latter passes dorsally and continues to enlarge, 

 becoming considerably dilated just in front of the heart ; from 

 this point it runs posteriorly, and gradually diminishes in size, 

 finally opening into the super-branchial chamber above the adductor 

 muscle. The hind-gut is characterized by the possession of a 

 longitudinal infolding of its ventral wall, forming what is known 

 as a typhlosole." 



" The Nervous System : The only point of interest connected 

 with the nerves is the presence of well-developed pedal ganglia ; 

 this, in face of the entire suppression of the foot, suggests that 

 the latter has only recently been aborted. The cerebro-pedal 

 commissures are, however, somewhat short. Owing to the great 

 transverse width of the mouth, the cerebral ganglia are widely 

 separated, and their transverse connective is consequently long." 



