ZOOLOGY. 575 



pass over into the succeeding circle. Their body there- 

 fore is only an homogeneous, transparent mass, variously 

 excavated and perforated, but not separated into two 

 cysts, namely, tegument and intestine, and without the 

 other viscera, which are formed by the vascular system, 

 such as the liver, kidneys, and salivary glands. 



SECOND CIRCLE. VASCULAR, SEXUAL ANIMALS. 



3491. These animals will both traverse the three 

 classes of their circle, as also repeat the preceding three 

 classes, and they consequently divide into two orders 

 and six families. 



Fourth Class. 



Venous, Ovarial Animals Mussels. 



3492. The Mussels or bivalve Mollusca resolve them- 

 selves into two orders, according to the structure of their 

 mantle or respiratory sac. It is either closed in a 

 tubuliform manner, and opens posteriorly into two 

 mostly tubular-shaped respiratory apertures, and in front 

 has an orifice for the passage of the foot Camacea 

 (Lochmuscheln) ; or it is slit along its whole length 

 anteriorly ; and the pedal aperture is confluent with the 

 anterior and also indeed with the posterior respiratory 

 opening Ostracea (Spaltmuscheln.} 



The first order still represent the cystic form of the 

 Oozoa, and partly by tubuliform calcareous shells placed 

 around the two ordinary shell-valves ; but the second, 

 by the freer development of the organs, represent the 

 animals of their own circle. 



Order 1 . Protozooid Mussels Lochmuscheln. 



3493. These animals repeat the Mucus- animals; are 

 tubular in form, and mostly provided with two long 

 respiratory tubes. 



Fam. 1. Infusorial Mussels Sackmuscheln or Pholades. 



Cylindrical, with a tubuliform mantle almost entirely 

 closed and frequently surrounded by a calcareous tube, 

 in addition to the two ordinary shell-valves ; the pedal 

 aperture is situated at the oral extremity ; the respiratory 



