8/4 SPONGES AND ZOOPHYTES 



microscopic examination, being small enough to be viewed entire in 

 the zoophyte-trough. There are few parts of the coast on which they 

 may not be found, especially on a calm warm day, by skimming the 

 surface of the sea with the tow-net ; and they are capable of being 

 stained and preserved in cells after being hardened by osmic acid. 

 The history of the large and highly developed Medusce 1 or ACA- 

 LEPH/E which are commonly known as 'jelly-fish ' is essentially simi- 

 lar ; for their progeny have been ascertained to develop themselves 

 in the first instance under the polype form, and to lead a life which 

 in all essential respects is zoophytic ; their development into Medusae 

 taking place only in the closing phase of their existence, and then 

 rather by gemmation from the original polype than by a metamor- 

 phosis of its own fabric. The huge Rhizostoma found commonly 

 swimming round our coasts, and the beautiful Chrysaora remarkable 

 for its long ; furbelows ' which act as organs of prehension, are oceanic 

 acalephs developed from very small polypites, which fix themselves 

 by a basal cup or disc. The embryo emerges from the cavity of its 

 parent, within which the first stages of its development have taken 

 place, in the condition of a ciliated ' planula,' of rather oblong form, 

 very closely resembling an infusory animalcule, but destitute of a 

 mouth. One end soon contracts and attaches itself, however, so as 

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

 four tubercles sprouting around it which grow into tentacles ; whilst 

 a slit in the midst of the central cells gives rise to the cavity of the 

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

 many additional tentacles ; and this, according to the observations 

 of Sir J. G. Dalyell on the Hydra-tuba, which is the polype stage of 

 the Chrysaora and other jelly-fish, leads in every important particular 

 the life of a Hydra ; propagates like it by repeated gemmation, so 

 that whole colonies are formed as offsets from a single stock ; and 

 can be multiplied like it by artificial division, each segment develop- 

 ing 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 when the time comes for the development of 

 its sexual gonozooids, the polype quits its original condition of a 

 minute bell with slender tentacles (fig. 664), assumes a cylin- 

 drical form, and elongates itself considerably ; a constriction or 

 indentation is then seen around it, just below the ring which encircles 

 the mouth and gives origin to the tentacles ; and similar constrictions 

 are soon repeated round 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 fieshy bulb, a (fig. 664, II), somewhat of the form of the 

 original polype, being still left at the attached extremity. The 

 number of circles is indefinite, and all are not formed at once, new 

 constrictions appearing below, after the upper portions have been de- 

 tached ; as many as thirty or even forty have thus been produced in 

 one specimen. The constrictions then gradually deepen, so as to divide 

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



1 See Professor Glaus, Untersuchungen iiber die Organisation undEntwickelung 

 der Medusen, Prague and Leipzig, 1883, and Miss Ida H. Hyde, ' Eiitwickelungs- 

 geschichte einiger Scyphomedusen,' in Zeitsclir. f. wiss. Zool. Iviii. p. 531. 



