3490. Hence, in these three classes there are found no more than three families, which together make up only one order. The want of a fourth family and so on, proves that the development of these animals does not pass over into the succeeding circle. Their body therefore 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 themselves 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.