Respiration in Invertebrate Animals. 47 



carried from the anterior to the posterior extremity of the shell 

 in any Acephalan, divides the mantle and the body into two very 

 distinct and dissimilar halves. On one side lie the branchiae and 

 extra-branchial, ventral, or oral chamber (PI. II. fig. 13) ; on the 

 other are disposed the viscera, the intra-branchial or dorsal 

 cavity; with the latter the exhalent or intra-branchial siphon 

 is necessarily and invariably connected ; in this dorsal compart- 

 ment, also, the anal orifice terminates. That cavity (b) which 

 lies on the ventral (the side opposed to the hinge) or right side 

 of the hypothetical line, whether the ventral borders of the 

 mantle be open or closed, siphonal or asiphonal, is always and 

 necessarily filled with pure water. In this chamber the branchiae 

 (PI. I. fig. 7 2 , a, b) whatever be their number or position, figure 

 or size, freely float ; it is here always that the oral orifice 

 (fig. 13, a) opens ; it is at once a reservoir of pure water for 

 breathing and pure material for food. All varieties centre in 

 the unity of this idea — all specific aberrations are reducible 

 to this basilar type. Specific diversities arise more frequently 

 from variations in the number, size, siphonal or non-siphonal 

 character of the openings communicating with this (oral or 

 extra-branchial) chamber (PI. I. fig. 6, b, b; fig. 7, c, d), than 

 from those which occur in the siphonal processes of the intra- 

 branchial or anal cavity (fig. 13, e). Mr. Clark * and Dr. J. E. 

 Gray t are the most recent and distinguished conchologists 

 who have attempted intelligently to found a classification of 

 the Conchifera on the basis of the varieties which occur in the 

 pallial orifices. Dr. Gray groups the whole class under two 

 primary designations — the Sipho?iophora and Asiphonophora — 

 which are again subclassified into orders, genera, and species. 

 In the Pholadidae, Myadae, Gastrochaenidae, and Solenidae, the 

 ventral borders of the mantle are united, and the siphonal tubes 

 are long and more or less distinct. The mantle is also closed 

 in the Corbulidae and Anatinidae, but the siphons are short. In 

 the Tellinidae the mantle is open, while the tubes are prolonged. 

 An open mantle coexists with short siphons in Cardiadae, Vene- 

 ridae, Mactridae, and Donacidae. An open mantle is co-present 

 with sessile tubes in Cycladidae, Kelliadae, Lucinidae, Cyprinidae, 

 Unionidae, and Arcadae. In Mytilidae, Ostreadae, Pectinidae, and 

 Anomiadae, the whole gape of the mantle is one undistinguished 

 capacious orifice. Guided by the rule that pure water must in 

 some manner or other, with adequate freedom, be admitted into 

 the oral or extra-branchial cavity, it is quite obvious that the 



* Ann. and Mag. Nat. Hist., June 1851. "On the Classification of the 

 Marine and Testaceous Mollusca." 



t Ann. and Mag. Nat. Hist., May 1854. 



