94 ZOOLOGY. 
water-vascular tubes, and are developed locally, asin Cestum, 
or along the whole length of the tubes, the sexually-differ- 
ent glands being placed in Beroé and allies on opposite 
sides of the tube. 
When ripe the eggs pass into the perivisceral space, and 
finally pass out through the openings of the body. The 
eggs of Pleurobrachia escape singly ; in Bolina they are 
Jaid in strings, while those of Idyia are deposited in a thick 
slimy mass. They spawn late in the summer and in the 
autumn. The young develop in the autumn, becoming 
nearly mature in the following spring. Development is di- 
rect, the young hatching nearly with the form of the adult, 
there being no metamorphosis. 
The species are widely distributed, a number being com- 
mon to both sides of the Atlantic, and the same species, ap- 
parently, of Plewrobrachia and Idyia occur on the east and 
west coast of North America. The most widely distributed 
forms are the Beroids. While the genus Mertensia is en- 
tirely arctic, the larger number of species are either tropi- 
cal or subtropical. The classification of the group is shown 
in the following summary. 
Cuass ITI.—CTENOPHORA. 
Spherical or oval, somewhat bilateral, scarcely radiated animals, with 
jelly-like, transparent bodies. The digestive tract opens at the posterior 
end into the perivisceral cavity ; from the canal pass off eight water-vas- 
cular tubes, which are in close relation with eight vertical meridional series 
of comb-like locomotive organs. Usually a pair of tentacles, which may 
become withdrawn into sacs, and are provided with thickset lasso-cells on 
the tentacular fringes. Nervous system consisting of an aboral ganglion, 
sending off eight nervous filaments to each of the eight rows of paddles. 
The sexual glands seated in the same individual. No metamorphosis, 
the young when hatched resembling the adult. 
Order 1. Hurystomee.—Body oval, with a large mouth and capacious 
stomach. The water-vascular tubes connected with the 
ctenophores, and forming numerous ramifications, commu- 
nicating by means of a circular canal near the mouth 
(Beroé, Idyia). 
