468 



ZOOPHYTES. 



as to form a foot ; the other enlarges and opens to form a mouth, 

 four tubercles sprouting around it, which grow into tentacula ; 

 whilst the central cells melt down to form the cavity of the 

 stomach. Thus a Hydra-like polype is formed, which soon 

 acquires many additional tentacula ; and this, according to the 

 observations of Sir J. G. Dalyell, leads in every important par- 

 ticular the life of a Hydra, propagates like it by repeated gem- 

 mation, so that whole colonies are formed as oft'sets from a 

 single stock, and can be multiplied like it by artificial division, 

 each segment developing itself into a perfect Hydra. There 

 seems to be no definite limit to its continuance in this state, or 

 to its power of giving origin to new polype buds ; but under 

 conditions not yet ascertained, the Strohila (as it is termed) 

 ceases to propagate by ordinary gemmation, and enters upon an 

 entirely new series of changes. In the first jDlace, the body 

 becomes more cylindrical in form than it previously was ; then a 

 constriction or indentation is seen around it, just below the ring 

 which encircles the mouth and gives origin to the tentacula ; and 

 similar constrictions are soon repeated around the lower parts of 

 the cylinder, so as to give to the whole body somewhat the 

 appearance of a rouleau of coins ; a sort of fleshy bulb, some- 

 what of the form of the original polype, being still left at the 

 attached extremity (Fig. 226, a). The number of circles is 



Fig. 226. 



SuccessiTe Stages of Development oC Medusa buds from Strotila larva :—a, polype body; b, il- 

 original circle of tentacula; o, its secondary circle of tentacula; d, proboscis of most advanced 

 Medusa disk ; e, polype bud from side of polype body. 



indefinite, and all are not formed at once, new constrictions 

 appearing below, after the upper portions have been detached ; 

 as many as 30 or even 40 have thus been produced in one speci- 

 men. The constrictions then gradually deepen, so as to divide 

 the cylinder into a pile of saucer-like bodies ; the division being 

 most complete above, and the upper disks usually presenting 

 some increase in their diameter : and whilst this is taking place, 

 the edges of the disks become divided into lobes (b), each lobe 

 soon presenting the cleft with the supposed rudimentary eye 

 (more probably an auditory organ) at the bottom of it, which is 



