632 
ZOOLOGICAL LITERATURE. 
The monograph is divided into three parts. The first (pp. 52) , 
entitled On the Anatomy of Synapta digitata'^ after a gene- 
ral introduction^ reviews the organization of the adult animal 
under the heads (1) internal skeleton^ (2) perisome, (3) alimen- 
tary canal and vascular system, (4) water- vascular system, (5) 
nervous system and organs of sense, and (6) reproductive organs. 
The subject-matter of each of these is still further divided, in a 
way very convenient for reference. 
The second part of the monograph (pp. 60) treats of ^^The 
Metamorphosis and Development of digitataf^ The author 
describes the method which he was led to adopt for procuring 
the young of this species — a minute, worm-shaped organism 
dwelling in hue mud at the bottom of the sea, and, by reason of 
this peculiarity of habit, escaping the notice of previous observers. 
These young are developed from one of the two kinds of Auricu- 
lariae described by Muller. Baur shows that the Auriculariae 
with calcareous wheels belong to S. digitatay and has examined 
vermiform young in which similar calcareous corpuscles were 
still clearly perceptible. He has observed all the essential inter- 
mediate stages, including the pupae which the auricularian 
larvae produce. Our author has also obtained the young of 
Synapta inhcerenSy now almost universally admitted to be distinct 
from S. digitata. To this section is appended an essay on , the 
so-called alternation of generations among the EchinodermatUy 
the nature of which Baur rightly apprehends, regarding it as a 
mode of internal development accompanied with metamor- 
phosis. 
The third and concluding division (pp. 120) of the work, by 
far the most voluminous, is devoted solely to a statement and 
partial explanation of the circumstances which attend the pre- 
sence of the snail-producing tube in some specimens of S, 
digitata, Baur in the first place points out the questions to be 
solved, and the attempts hitherto made with that object. He 
then proceeds to demonstrate that the molluscigerous sac is an 
animal organism complete in itself, living for itself, and provided 
with sexual apparatus — in short, a sac-shaped, naked Mollusk, 
which he would name Helicosyrinx parasitica. The production 
of several successive broods, which undergo their complete deve- 
lopment within the sac, excludes the hypothesis of its being a 
mere proglottis or generative segment (p. 58). The place for 
this Mollusk would seem to be among the Apneustay or near the 
Nudibranchiata. Though destitute of feet, shell, and gills, the 
structure of its generative organs and young clearly show that 
it belongs to the class of Gasteropoda. Helicosyrinx possesses 
the following parts : — body- wall, digestive canal, ovaria and 
testes, accessory reproductive glands, and generative chamber. 
There are two orifices, an oral and a genital. No anu^ exists. 
All these parts are described in detail. They are organically 
