The Salpidæ are another family of free-swimming Ascidians. The tunic is perfectly hyaline, the body is somewhat cylindrical, but compressed and open at both ends ([fig. 163]). The mouth is a slit, the discharging orifice is tubular and can be opened and shut. The breathing apparatus is in the form of a ribbon extending obliquely across the cavity of the tunic, the ear with four otolites is in the ventral fold, and the flux of the pale blood is alternate as in other Tunicata.
Fig. 163. Salpa maxima.
Fig. 164. Young of Salpa zonaria.
The Salpidæ are produced by alternate generation. A solitary floating Salpa is always found to contain a chain of embryos joined end to end winding spirally within her. They are all of one size, and portions are liberated in succession through an aperture in the tunic. In a little time these connected larvæ are developed into a chain of adult Salpæ. The individuals are from half an inch to several inches long, according to the species, and when joined end to end the chain may extend many feet, but the attachment is so slight that they often break up into shorter portions. The chains swim with an undulating serpentine motion either end foremost by the simultaneous expulsion of water from the muscular tunic of each individual. A single egg is formed by each of these creatures, which remains within the parent till a solitary Salpa is hatched, and then it comes into the water, and after a time produces a chain of larvæ.
The aggregate young of the Salpa zonaria, instead of being united end to end, are applied side to side, and as the individuals are broad at one extremity and narrow at the other, they constitute groups continually diminishing in size, which take a spiral form.
The reproduction of the whole genus of Salpidæ is rapid and enormous. Dr. Wallich mentions that while sailing between the Cape of Good Hope and St. Helena, the ship passed for many miles through water so crowded with the Salpa mucronata that it had the appearance of jelly to apparently a great depth. The Salpæ, which were from one to two inches long, had yellow digestive cavities, about the size of a millet seed, which contained diatoms, Foraminifera, Polycystinæ, small shrimps, and other microscopic creatures.
SECTION XI.
MOLLUSCA.
Although the Mollusca do not come within the limits of this work they nevertheless afford objects worthy of microscopic investigation. The gills of a bivalve mollusk are like crescent-shaped leaves fixed by their stalks to the transverse extremities of the mantle, so that the greater part floats freely in the water.