18 COMPARATIVE ANATOMY. 



valved shell, arranged antero-posteriorly to body. Respira- 

 tory apparatus in the form of two long, ciliated tentacles. 

 Alimentary canal curved upon itself, and either ends blindly 

 in the middle line or else terminates in a distinct line be- 

 tween lobes of mantle. (Huxley.) Nervous ganglion placed 

 within the re-entering angle of intestine. Vascular system 

 imperfectly understood. An atrial circulation, or movement 

 of water within system of communicating chambers, is well 

 developed. 



Tunicata. Compound or single molluscs. Respiratory 

 apparatus included within mantle. Gullet commences from 

 the base of this branchial chamber. Endostyle* present. 

 The intestine is curved upon itself and terminates at anal 

 aperture, which is always situated within a chamber opening 

 externally by a second aperture upon mantle. The nervous 

 ganglion not placed within entering angle of intestinal fold. 

 Heart of simple form always present. Atrial system present 

 as in Brachiopoda. 



Lamellibranchiata. Solitary molluscs. Shell bivalved, 

 arranged to right and left half of mantle. Respiratory ap- 

 paratus in the form of lamellae and are four in number, two 

 on either side of visceral mass. Mouth often furnished with 

 labial lobes. The intestine generally penetrates the heart, 

 and terminates near anus between the mantle lobes at pos- 

 terior extremity of body. Nervous ganglia generally of three 

 pair, arranged as follows : one for the parts about the mouth, 

 one for the foot, when present, and the remaining one for the 

 posterior adductor muscle. The heart, often of two cham- 

 bers, occasionally of one. 



Gasteropoda. Solitary molluscs. Shell univalved or mul- 

 tivalved; whorls dextral, as a rule. Respiratory appara- 

 tus; comb-like gills in aquatic, lung-like structure in terres- 



* The middle of the haemal wall, viz., the side toward the heart, the pharynx, 

 is pushed into a longitudinal fold, the bottom of which projects into a blood 

 sinus, and has a much thickened epithelial lining. Viewed from one side the 

 bottom of the fold consequently appears like a hollow rod, and has been 

 termed the "endostyle." The functions of this structure are unknown. 

 (Huxley.) 



