727 
ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
monograph is one of the kind that is of great value in the present state 
of zoology. 
New and Primitive Type of Compound Ascidian.*— -Mr. W. Gar- 
stang describes an interesting form of compound Ascidian which he 
dredged off Plymouth in 5 to 15 fathoms. He calls it Archidistoma 
(A. aggregatum ) and defines it as having incrusting colonies, and con- 
sisting of a spreading basal portion from which zooids arise at irregular 
intervals. Zooids free and partially fused, with distinct oral and cloacal 
apertures. No incubatory diverticulum of the cloaca. The test is 
arenaceous and there are about thirty tentacles ; the ova are large and 
contain much food-yolk. 
Archidistoma is a connecting link between the true Distomidae and 
the Clavelinidae. No true Distomid is known to possess free zooids, 
that is, zooids not completely imbedded in a common test. This new 
form, therefore, combines the structural characters of the Distomid*e 
with a social form of colony which is only slightly removed from that of 
the Clavelinidae. It is also of especial interest because it exhibits tbe 
first stage in the evolution of the coenobitic type of colony from the 
social Ascidian type, in which the zooids are entirely free and irregu- 
larly placed. Though the clumps of its zooids have no common cloaca, 
the cloacae of the individuals are usually situated towards the centres of 
the groups. 
£. Polyzoa. 
Freshwater Polyzoa.f — Dr. F. Braem begins his memoir with a 
faunistic account of the freshwater Polyzoa of Prussia, where all the 
species known to be native in Europe are represented. He then passes 
to the problems connected with the development and reproduction of the 
Phylactolaemata. In Cristatella — and the same is true for the others — all 
the buds of the colony are traceable to a limited complex of embryonic 
cells, left over from the material of the statoblast or of the ovum, and 
carried on from bud to bud. This relation is expressed as the “ principle 
of the double-bud,” each bud usually forming, on its oral aspect and 
directly from itself, two daughter-buds, which multiply in the same 
way. In youth more than two buds may be formed ; in older stages 
sometimes only one. The cystids — portions of the colonial wall inter- 
polated between the polypides — also develope from the cells of the 
polypoid rudiment. The varied growth of the stock is discussed in 
detail. Besides the budding of individuals, there is budding of the entire 
colony. The movement of the Cristatella-stock is an essential condition 
of its growth. Dr. Braem then describes the development of the indi- 
vidual, the formation of the funiculus, and the origin of the statoblasts. 
He is convinced that the statoblasts are derived from both layers of the 
budding which leads to the formation of the germinal cells in the funi- 
culus. The formation of statoblasts is like that of the buds ; all are 
referable to older buds, which from the first divide into material for the 
upbuilding of the colony and material for the continuation of the species. 
The author then discusses the environmental conditions— such as cold — 
* Ann. and Mag. Nat. Hist., viii. (1891) pp. 265-8 (2 figs.). 
t Bibliotheca Zool. (Leuckart and Chun), vi. (1890) 134 pp., 15 pis. and many 
figs. 
