112 COMPARATIVE ANATOMY CHAP. 



is always obscured, and often so much so as to be unrecognisable. 

 The following different kinds of such sterile persons may be dis- 

 tinguished : 



a. Persons in which the following typical organs of a Medusa may 

 still be recognised : (1) a variously-shaped protective or braet as 

 metamorphosed umbrella. It serves as umbrella or shield, and affords 

 protection, not only to the other parts of the same person, but also 

 to the neighbouring persons, which can withdraw under it. (2) The 

 oral or gastric peduncle (siphon), the chief organ for taking in food 

 and digestion. The siphon is often stalked, and the edge of the 

 mouth widened into a funnel, or produced into 4 points, or prolonged 

 like a proboscis. (3) A very contractile tentacle or capturing filament, 

 which is placed at the base of the gastric tube. The tentacle is 

 feathered on one side, i.e. it is provided with one row of lateral 

 branches, whose ends are armed with stinging batteries. Such a 

 sterile person simultaneously performs the functions of taking in food 

 and of protection (Fig. 83, A}. 



b. Persons distinguished from those just described by the fact that 

 the contractile hollow siphon has lost its mouth, and so appears 

 changed into a taster or feeler (palpons). The tentacle at the base of 

 the feeler becomes an unfeathered, long, and very retractile sensory 

 filament (Fig. 83, B). 



c. Persons in which the umbrella is completely degenerated, and 

 which consist of nothing but siphon and tentacle (Fig. 83, C). 



d. Persons which have retained exclusively the function of pro- 

 tection, and in whom the umbrella alone, in the form of a bract, has 

 attained development, while the formation of siphon and capturing 

 filament has been suppressed (Fig. 83, E). 



e. Persons reduced to tasters, without bracts and without sensory 

 filaments, 



C. Special swimming-bells. Nectophores, agreeing in structure 

 with the ordinary swimming bells developed at the upper end of the 

 stem are found in some Siphonanths on other parts of the stem as well. 



These various appendages, or heteromorphic persons (A-C), of 

 which several may be wanting, occur in different and often very 

 characteristic order and manner of division on the stem. They are, in 

 the first place, arranged in many Siphonanths in distinct groups, 

 repeated at regular intervals and separated by internodes of the stem. 



The following are the chief modifications which occur in the com- 

 position of such a group, which is known as a eormidium : 



A. The eormidium consists of (1) a gonophore and (2) a sterile 

 person with bract, siphon, and capturing filament (Fig. 86). 



B. To these two persons a third person, a special swimming-bell, is 

 added. 



C. The eormidium consists of (1) one or more gonophores, (2) one 

 sterile person with siphon and tentacle, but without bract. . 



