102 >\f '-CLASS in. 



finished: the halves again divide in a direction perpendicular to 

 the former line of division : but here the fission does not always 

 begin in the stomach. How often the process may be repeated is 

 not known. No trace of fission was observed in Mesonema, though 

 it was often seen to occur in the larval forms, even when ova were 

 already distinctly visible on the vessels.] 



[The Siphonophorm have been shewn, by the investigations of 

 HUXLEY, LEUCKART, KOELLIKER, GEGENBAUER and VOGT, to be 

 compound animals, or colonies, connecting the hydroid polyps with 

 the acalephs. They are named by KOELLIKER, in consequence, 

 Swimming Polyps (polypi nechalei). They consist in general of a 

 stem, usually cylindrical and long (Diphyes), sometimes shortened 

 and sacciform (Physalia), sometimes disciform (Velella), to which 

 appendages are attached which differ remarkably in form and func- 

 tion. Some of these are suctorial tubes or stomachs, others motive 

 organs, others feelers and prehensile organs, others again protective 

 laminae (bracts) and sexual capsules. Great differences prevail with 

 respect to the number, arrangement, and development of these parts, 

 in the different families : those which are constant in all siphono- 

 phors are the stomachs, the prehensile apparatus, and the sexual 

 capsules. The stem is muscular, and hollow the interior forming a 

 canal in which the nutrient fluid moves with rapidity. The swim- 

 ming apparatus is either passive or active when passive it is a 

 hydrostatic apparatus consisting of a bladder filled with air which is 

 always placed at the upper extremity of the common stem : when 

 active it consists of swimming-bells, which are also placed at the 

 upper extremity of the stem, and are variously grouped, and in 

 variable number in different genera : the swimming-bells may exist 

 conjointly with the air-sac or without it. These bells are, in 

 general, formed on the plan of a Medusa, consisting of an elastic 

 bell-shaped mantle, very various in form, with an internal mus- 

 cular- layer which surrounds the swimming-sac. On the outer sur- 

 face of the latter, there is a system of four radiating vessels, which, 

 at the circumference of the aperture, fall into a circular vessel, and 

 at the summit of the bell arise from a single vessel, which passes 

 through the pedicle of the bell and falls into the cavity of the com- 

 mon stem. All the other appendages of the stem have also a more 

 or less perfect system of vessels, which communicate with the inter- 

 nal cavity of the stem in a similar way. The only communications 



