690 The American Naturalist. [August, 
RECENT LITERATURE. 
The genus Salpa.—The Johns Hopkins Press has issued the 
second of the series of * Morphological Monographs," in the shape of 
a magnificent treatise on the genus Salpa by Professor W. K. Brooks. 
The monograph is an exhaustive one, without which no working 
library can afford to remain. It includes a brief but valuable survey 
of the anatomy of many species, a detailed account of the develop- 
ment of the solitary form from the egg, and of the chain Salpa from 
the stolon. The systematic position of Salpa with reference to other 
tunicates is discussed, and this leads the author to a wide biological 
consideration of the primitive pelagic fauna and the origin of the 
Metazoa. The evidence on the origin of the Chordata, to be gathered 
from the tunicates, is presented and is shown to be in opposition to the 
annelidian hypothesis of the derivation of this group. Dr. M. M. 
Metcalf contributes the final section, a careful study of the eyes and 
subreural gland of Salpa. 
The chapter on the egg development of the solitary Salpa is espe- 
cially interesting and suggestive. An outline of this remarkable 
development is as follows; The germ mass is present in the embryo 
of the solitary form, and extends into the stolon as the latter grows 
out. It is differentiated into a superficial epithelium and an inner 
mass of ovarian ova, which in the mature stolon form a single row. 
When the stolon is constricted to form the chain of salps, each Salpa 
body gets its particular portion of the elongated germ mass. In most 
species this consists of a single egg with its surrounding epithelium. 
The latter is differentiated into testes, follicle, and fertilizing duct, i. e. 
a tube attaching the egg to the dorsal wall of the chain salp, through 
which the spermatozoa pass to reach the egg—the egg itself liesin a 
blood sinus of the chain salp. It is evident from these facts that the 
alternation of generations in Salpa differs from the typical alternation 
of generations, in that the sulitary form does not arise from the chain 
Salpa, but from an egg passed into the chain Salpa from the preceding 
generation of the solitary form. 
As the embryo grows, it pushes out of the blood sinus in which it lies 
at first, into the cavity of the cloaca, driving the wall of the cloaca 
before it. From the dorsal wall a complicated system of covering 
"The genus Salpa, by William K. Brooks. Baltimore, The Johns Hopkins 
Press, 1893. 
