PREFACE. ix 



individuals separate, and after expelling portions of the broken-up nuclear 

 structures (probably as effete products), proceed to re-form the nucleus, 

 or nucleus and nucleolus characteristic of the species. The so-called Acineti- 

 form embryos appear to be parasites, the rod-like bodies occasionally observed 

 in the nucleus are also parasites, whilst the striated structure and spindle- 

 shape exhibited by the nucleolus or paranucleus in such forms as Para- 

 moecium and Stylonichia at the period of conjugation, are simply due to 

 changes in this body which are exactly paralleled in the nuclei of egg-cells and 

 other tissue-elements of multicellular organisms, when those cells are about 

 to divide by transverse fission. The process of conjugation in the Infusoria 

 may be, and probably is, attended by an exchange of nuclear material 

 between the conjugating individuals, and is so far comparable to sexual 

 congress, but it results in a simple " rejuvenescence " of the conjugating 

 individuals and not in a production of spores. Reproduction by fission and 

 by the modification of fission, known as gemmation, has been accurately 

 observed in Infusoria, but of the formation of " spores " in this group 

 Ave are at present ignorant, in spite of all that has been written on the 

 subject. 



Origin of Male and Female Reproductive Elements from 

 different Germ-layers. — In § 95 Professor Gegenbaur has described 

 the observations of Ed. Van Bexeden on the development of the sexual 

 products in Hydractinia, and has adopted his generalisation, so far at 

 least as it applies to the Hydromedusa\ From more recent observa- 

 tions (Ciamiciax, Zeitschr. firr wiss. Zoologie, vol. xxx. p. 501, 1878) 

 it appears that in other genera of hydroid polyps the same arrangement 

 does not obtain. In Eudendrium ramosum the ova appear to develop from 

 the ectoderm, and the sperm from the endoderm ; in Tubularia mesem- 

 bryanthemum both ova and sperm are ectodermal in origin according to 

 Ciamician ; Van Beneden found the ova to be endodermal and the sperm 

 ectodermal in Hydractinia, whilst Kleinenberg ascribes both to the ecto- 

 derm in Hydra. 



Nervous System and Sensory Organs of Medusa?. — During the 

 past year a considerable addition has been made to knowledge on these 

 points, by the researches of the two Hertwigs (" Das Xerven system und die 

 Sinnesorgane der Medusen." Leipzig, 1877). It is no longer possible to 

 deny the existence of differentiated nervous tissue in the Medusa 1 — the 

 central organ having the form of a ring situated along the line of insertion 

 of the velum in the Craspedota, and of a series of isolated ganglia, usually 

 eight in number, placed on the edge of the disc in the Acraspeda. (See for 

 an abstract of recent researches on this subject, Quart. Journal of Microsc. 

 Science, vol. xviii. p. 310.) 



Cirri and Elytra of Aphroditacerc. — The statement in § 105, that 

 the elytra of the chsetopodous Worms, allied to Aphrodite, are formed by the 

 metamorphosis of the dorsal cirri of the parapodia, appears to be contradicted 



