94 ZOOLOGY. 



water- vascular tubes, and are developed locally, as in Cesium, 

 or along the whole length of the tubes, the sexually-differ- 

 ent glands being placed in Beroe 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 

 laid 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 Pleurobrachia 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. 



CLASS III. 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 become withdrawn into sacs, and are provided with thickset lasso- cells on 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, 

 tJie young when hatched resembling the adult. 



Order 1. EurystomecB. 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 

 (Beroe, Idyia). 



