560 HISTORY OF SYSTEMS. 



Subdivision II. Cloak more or less closed, forming 

 siphons. 

 Tribe I. One siphon. Mytilusidae, Uniodae, Cardita, 



Venericardia, and Crassatella. 

 Tribe II. Cloak closed posteriorly and anteriorly, 



forming three apertures. Tridacna and Hippopus. 

 Tribe III. Anterior opening large. Chama, Car- 



dium, Donax, Tellina, Venus, Mactra. 

 Tribe IV. Anterior opening small. Mya, Solen, 



Pholas, Teredo. 



Section II. ACEPHALA TUNICATA. 



Subdivision I. Interior tunic detached from the external 

 one 9 and united only at the two orifices. 

 Tribe I. Body permanently fixed to other bodies. 



A. Simple Animals. 



1. Apertures furnished with four rays. Boltenia, 

 Cynthia, Csesira, Styela, Pandocia. 



2. Apertures with indistinct rays of more than 

 four. Clavelina, Pirena, Ciona, Phallusia. 



B. Compound animals. 



1. Branchial orifice radiated. 



a. Branchial and anal orifices with six rays. Diazona, 



Polyzona, Sigillina. 



b. Branchial orifice only furnished with six rays. Sy- 



noicum, Sydneum, Polyclinum, Aplidium, Didem- 

 num. 



2. Branchial orifice simple. Botryllus. Euclaeium. 

 Tribe II. Body free and moving about in the water. Py- 



rosoma. 



Subdivision II. Interior tunic adhering throughout to the 

 external one. 

 Salpa. 



I have next to lay before you the " Natural Arrangement 

 of Mollusca according to their internal Structure," proposed 

 by John Edward Gray. (March, 1821.) * 



Sub-kingdom MOLLUSCA, Cuv. 



Animal without any bony skeleton ; muscles attached to 

 the skin ; skin soft, not articulated nor annulated ; nervous 

 system irregular. 



* London Medical Repository, xv. 229 239. 



