112 SYNOPTIC COLLECTION. 



brachia rhododactyla Ag. (No. 152), Cesium veneris Less. 

 (No. 153), Idyia roseola Ag. (No. 154), and Beroe ovata 

 Esch. (No. 155). 



Pleurobrachia rhododactyla Ag. (No. 152), is frequently 

 seen off the New England coast. Its transparent body is 

 spherical, with eight rows of comb-like structures or plates 

 extending from pole to pole. These are, in reality, cilia 

 which have become united, as shown by the development 

 of the animal. They constitute the peculiar characteristic 

 of this group, giving it the name Ctenophora. The ex- 

 tremely long tentacles may extend from the body or else 

 be tucked away out of sight in two lateral pockets. It 

 has been found that another Ctenophoran, Bolina, is so 

 similar to Pleurobrachia when it leaves the egg, that one 

 cannot be distinguished from the other except that the 

 compression of the body in Bolina is in a plane at right 

 angles to that of Pleurobrachia. The postembryonic de- 

 velopment, however, produces marked changes of form, 

 complex windings of vessels, and the almost complete 

 disappearance of tentacles which are at first developed 

 like those of Pleurobrachia. 



Cesium veneris Less. (No. 153), is instructive on ac- 

 count of its phylogenetic relations to other Ctenophora. 

 There are few groups of the animal kingdom where the 

 postembryonic metamorphosis so strikingly recapitulates, 

 even in the details of organization, the adult forms of 

 more simply organized groups, as do the larval stages of 

 Cestum and the lobed Ctenophora recapitulate the adult 

 stages of the generalized Ctenophora. 1 



The adult Cestum has distinct bilateral symmetry. Its 

 long, belt-like appearance has won for it the name of 

 Venus's girdle. Eight rows of plates or combs extend 

 longitudinally down the body, and these aid in locomotion. 

 The mouth is near the middle of the belt-like body and 

 possesses two tentacles which extend from a pocket. 



1 Allman, On the Development of Ctenophora, Journ. Linn. Soc. 

 London, Zool., XVI, 1882, p. 106. 



