ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 445 



finally " evolves itself into a medusiform body or 6 -hooked brood. These 

 six hooks, placed at equal distances, support the velum from the interior 

 of the invaginated gastrula cavity." The author thinks his observations 

 give support to the view that Cestodes have " possibly never advanced 

 beyond a gastrula stage in their development." 



New Species of Tapeworm.* — T. B. Eosseter describes Drepanido- 

 tcetiia sagitta sp. n., from Anas boschas. The scolex is pyriform and 

 perforated at its base by a circular orifice. This is a naturally formed 

 inverted cavity from whose sides spring the muscles which elevate and 

 retract the rostellum. This organ is long, flexible, attenuated, and armed 

 with sickle-shaped hooks. There are four oval suckers so placed around 

 the scolex as to give the head an arrow-like appearance. The most dis- 

 tinctive internal feature appears to be the " spatuliform monotestis." 



Some New and Little Known Trematodes.f — W. Nicoll comments 

 on the neglect of the study of Trematodes in Britain, and offers some 

 contributions. He describes Echinostomum secundum sp. n. from the 

 intestine of Larus ridibundus and L. argentatus ; Zeugorchis acanthus 

 g. et sp. n., from the bursa fabricii and cloaca of L. argentatus, which 

 differs widely from Echinostomum ; Levinsenia similis {Spelotrema simile) 

 Jagerskold ; and Psilostomum redactum sp. n., from the intestine of 

 Gasterosteus aculeatus. 



Life History of a Trematode of the Oyster. J — D. H. Tennant 

 has elucidated some points in the early development of Bucephalus 

 haimeanus, a parasite of the oyster. Experimental evidence has shown 

 that B. haimeanus is a larval stage of Gast&rostomum gracilescens. Germ- 

 cells arise within the sporocyst wall of the latter, give off small cells 

 which may be regarded as polar bodies, segment and develop into the 

 cercaria of B. haimeanus. It is further shown in other cases that 

 supposedly different species are physiological varieties of the same species. 

 The presence of sporocysts and cercaria of G. gracilescens in the oyster 

 prevents the formation of reproductive elements, and also renders the 

 host incapable of withstanding adverse conditions. B. haimeanus 

 thrives best in oysters living in brackish water. Its growth is inhibited 

 by increased salinity. 



Incertee Sedis. 



Life-History of Orthonectids.§ — F. Mesnil and M. Caullery have 

 studied Rhopalura pelseneeri and compared its life-history with that of 

 Dicyemids. There is a phase of asexual multiplication in the host, in the 

 form of plasmodia, whence sexual individuals arise at the expense of 

 " cell germs." Then there is a phase of propagation, from one host to 

 another, secured by the sexual individuals which are ciliated and pro- 

 duce ova. These develop, doubtless after fertilisation, in the interior 

 of the body of the mother, into ciliated larvse, known only in 

 R. ophiocomce Geard and R. pelseneeri G. and M., but doubtless of 



* Journ. Quekett Micr. Club, 1906, pp. 275-8 (1 pi.). 



t Ann. Nat. Hist., xvii. (1906) pp. 513-27 (2 pis.). 



j Quart. Journ. Micr. Sci., No. 196 (1906) pp. 635-90 (4 pis.). 



§ Comptes Rendus, cxli. (1905) pp. 774-6. 



