ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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vacuolated cells ; this rudiment is invaginated, and the cells bear a strong 
tuft of cilia ; subsequently the groove is gradually reduced in size. 
The head-gland begins as an area of round ectoderm cells ; these 
become cylindrical, and the area is invaginated, increasing in Tetrastemma , 
dwindling in Drejpanojphorus. 
A thickening of ectoderm is invaginated to form the proboscis. In 
Tetrastemma the invagination divides, a dorsal portion forming the pro- 
boscis, a ventral portion forming the secondary stomodaeum which 
communicates with the oesophagus. 
The oesophagus arises as a group of much elongated cells, which are 
invaginated and form a tubular cavity. In Tetrastemma this is con- 
stricted off from the ectoderm, and communicates with the secondary 
stomodaeum. 
The rectum is formed like the oesophagus, and communicates with 
the diverticulum of the gut. 
The four primitive mesoderm cells, which lie before and behind the 
blastopore, divide karyokinetically, and each forms a mesoderm band, 
which differentiates into a somatic and a splanchnic layer. The paired 
anterior mesoderm bands subsequently form an anterior (ventral) 
mesoderm sack ; the paired posterior mesoderm bands form a posterior 
(dorsal) mesoderm sack ; the two sacks unite ; the splanchnic layer 
clothes the gut, the somatic joins the ectoderm ; the cavity between is 
the body-cavity. 
The mesoderm of the proboscis arises from two mother-cells, which 
lie at first at the margin of the proboscis invagination, but subsequently 
migrate into the segmentation cavity. Each divides karyokinetically and 
forms a mesoderm band. Both of these differentiate into two layers 
with a cavity between. The bands envelop the proboscis and unite, 
the inner layer joining the proboscis, the outer forming the proboscis- 
sheath, with the rhynchocoelome between. 
The brain arises from two pairs of ectodermic thickenings, the dorsal 
and ventral ganglia. The ventral longitudinal strands arise from two 
thickenings of ectoderm, each connected with the corresponding ventral 
ganglion by a slight ectodermic thickening. Behind the dorsal ganglia 
the dorsal longitudinal strands arise in a similar manner, but they soon 
cease to elongate, and finally coalesce with the dorsal ganglia. 
The cerebral organs arise as invaginations of the ectoderm. The 
ectoderm is gradually differentiated into covering, flask-like, and other 
elements. 
In a continuation of the paper the author compares his results with 
those of others, and discusses the systematic position of the Nemertines. 
New Metanemerteans.* — Dr. T. H. Montgomery, jun., describes 
some American forms, beginning with Zygonemertes virescens Yerr. g. n. 
( = Amphiporas virescens Yerr., 1892). The rhynchocoel extends to the 
posterior end of the body, while the thickened proboscis (with the exclu- 
sion of its retractor muscle) does not extend quite half the length of the 
rhynchocoel. The basis of the central stilet is very large, considerably 
elongated, flattened, or slightly concave posteriorly ; the basis is con- 
stricted near its posterior end. The central stilet is straight, massive, 
not half the length of its basis. There are ten or eleven longitudinal 
* Zool. Jakrb. (Abth. Syst.), x. (1897) pp. 1-14 (1 pi.). 
