20 A Guide to the Zoological Collections in the 



to form a ring of muscle, by the contraction of which, under 

 the control of a double ring of nerve-tissue, the Medusa 

 swims through the water, and along the edge of the bell 

 tentacles and sensory organs (auditory and visual) are 

 developed at the point where the radial canals join the 

 circumferential canal : these sense-organs are never cover- 

 ed over by folds of membrane. The development of the 

 Medusa from the ordinary nutritive-zooid type into the 

 free locomotive gonozooid type is well displayed in the 

 enlarged glass-models of Bougainvillea fmticosa, Syn- 

 coryne frutescens, Perigonimus vestitus^ etc., in Case 3. 



From such a typical Medusa-iorm two other modifica- 

 tions arise, one being of the nature of an advancement, 

 the other being probably a degradation. 



The advanced type of Medusa is shown in the beauti- 

 ful specimen of OUndias miilleri, a large Medusa which 

 throughout its life is known only as a free animal, and 

 never springs as a bud from a tree-like colony. The same 

 thing is illustrated in the series of models of Carmarina 

 has tat a. 



The degraded type of Medusa is seen in the exhibited 

 specimens of Sertularia, and in the beautiful spirit-speci- 

 men of Tubularia larynx^ where the reproductive units 

 of the polyp-colony, instead of breaking away and becom- 

 ing freely locomotive Medusae with mouth and tentacles, 

 merely grow into large buds or sacks, without mouth and 

 tentacles, in which the reproductive elements are developed. 



These arrested or incompletely developed Medusae are 

 usually known as " Medusoid gonophores." 



An intermediate step, in which the reproductive zooid 

 (gonozooid) becomes developed into a more or less perfect 

 Medusa, wliich, however, never becomes detached from the 

 parent colony, occurs, and is represented in the collection 

 by the enlarged models of Tubularia indivisa, in Case 3. 



In some Hydrozoa the physiological division ot labour 

 goes far beyond the simple separation of the units of the 

 colony into two classes of zooids, the nutritive and the re- 



