CHAPTER EIGHTH. 



SIPHONOPHOR^. 



SECTION I, 



SIPHONOPHORiE IN GENERAL. 



Modern zoologists generally consider the Siphonophorje as one natural group of 

 Acaleplis, which they have subdivided in various ways, into families or tribes. 

 When closely compared, it appears, however, that they differ widely, in a morpho- 

 logical point of view; while the characteristics by which they are held together, 

 are of the most trifling nature, consisting chiefly in the fact that they are free, 

 moving animals, and not attached to the ground. Kolliker has, nevertheless, 

 insisted upon that feature as essential, and on that account called them Polypi 

 nechalei. In attempting to classify them, I have kept in view the prominent 

 difference pervading the whole class of Acalephs, in which individuals assume either 

 the characteristics of attached Hydroids or of free Medusae, with every degree of 

 approximation to one or the other of these extreme forms. In Siphonophoras 

 the hydroid type is prevalent, but already raised above the ordinary condition of 

 Hydroids, in being free ; and the medusoid element is lowered, in so far as most 

 Medusae, budding from the colonies, are deprived of some of the characteristics of 

 the higher Acalephs. Moreover, hydroids and medusae, budding from one another, 

 invariably form polymorphous communities, from which various parts are cast off 

 to continue a short, precarious existence, as independent beings. The connection 

 of all these isolated members of the Siphonophorae, has only recently been traced 

 in a satisfactory manner. Upon the prevalence of the hydroid or medusoid 

 elements, and their various combinations among these Acalephs, aided by what is 

 ah'eady known of their development, I venture to subdivide the Siphonophorae 

 into four sub-ordei's : the Porpitae of Goldfuss, or ChondrophoriB of Chamisso and 



