No. 402.] REVIEWS OF RECENT LITERATURE. 525 
A biannulate condition of the leech somite has probably pre- 
ceded phylogenetically the triannulate condition ; still earlier the 
somite was probably uniannulate, as in the chetopods. The simple 
uniannulate somite became biannulate by the separation of a narrow 
posterior ring from the rest of the somite ; the triannulate condition 
was reached by the separation of a narrow ring at the anterior end 
of the somite, the sensilla remaining on the middle ring. 
These conclusions have been reached quite independently by 
Moore, who presents incontrovertible evidence in their support. He 
further expresses the opinion that the shorter somites commonly 
found at either end of the body of a leech are not, as they are 
usually regarded, “abbreviated” somites once multiannulate, but 
rather represent “stages of development arrested or in progress ” 
from the uniannulate to the multiannulate condition. 
The number of somites in the body of Microbdella is probably 
the same as in Glossiphonia (Clepsine) and Herpobdella (Nephelis), 
though Moore finds some evidence, not to his mind conclusive, of the 
existence of an additional somite at the anterior end of the body. 
Locomotion of Solenomya. — Solenomya and its relatives show 
three methods of locomotion which have been studied by G. A. 
Drew. The first is well represented in Yoldia. This clam pos- 
sesses a spatula-like foot split into two plates at its distal end. 
The animal drives this foot into the mud, with the distal flaps held 
together. "These are then expanded and serve as an anchor so that 
the contraction of the longitudinal muscles of the foot draws the 
animal through the mud to the place where the foot is anchored. 
There is no reason to suppose that these mollusks creep about on 
the expanded foot as snails do. The second method of locomotion, 
that of leaping, is seen in Solenomya, and especially in Yoldia. 
When the animal rests sidewise on a smooth surface, the foot is 
protruded and turned under the lower valve. If the foot is then 
suddenly contracted, the shell may be thrown end for end some 
inches. The third method of locomotion is that of swimming. 
This is accomplished by the vigorous ejection of water from the 
mantle cavity. The mantle lobes are united except at their anterior 
and posterior ends. By the separation of the valves, through the 
action of the elastic ligament, the mantle chamber is filled with water. 
The anterior opening is then closed by the foot, and by a vigorous 
2 Drew, G. A. Locomotion in Solenomya and its Relatives, Anat. Anz., Bd. 
xvii, pp. 257—266, 1900. 
