390 SUMMARY OF CUKRENT RESEARCHES RELATING TO 



Platyhelmintlies. The primitive cavity probably represents tbe space 

 whicli was once bounded by the cells of the mesenteron. 



In conclusion, the authors discuss the colonial nature of the Cestoda, 

 and the possibility of an alternation of generations. They find that the 

 cysticerci, with the exception of the echinococci, &c., undergo a simple 

 metamorphosis, which is often largely wanting. This may be made 

 clearer by a consideration of the case of T. elUptica ; in it the anterior 

 part of the six-hooked embryo forms the scolex, and the hinder part a 

 rudimentary organ, the tail, which, later, falls off. In other Tsenise this 

 hinder part is converted into a tail, but in them the hinder part of the 

 anterior portion forms an embryonic covering ; consequently, only the 

 anterior part of the anterior portion forms the scolex, aud it is by the 

 growth and segmentation of the scolex that the adult Tsenia is formed. 

 The authors do not believe in any asexual generation ; such does, of 

 course, obtain in Echinococci, but this is a secondary and adaptive 

 phenomenon. 



In the development of the organs it is observed that the six-hooked 

 embryo is formed from a blastema, or embryonal tissue which is not 

 differentiated into germinal layers ; from this the organs are developed 

 directly, and are at once found, with the exception of the rostellum, in 

 their definite positions. As we pass from without inwards in a young 

 cysticercoid we meet with a peripheral zone of subcuticular cells, a 

 parenchyma in which the cells are closely packed, and a soft parenchyma 

 in which the cells are scanty ; and between these three there are no 

 definite boundaries. The authors suppose that the nervous system and 

 the water-vascular apparatus are developed from the closely packed 

 parenchyma, with the exception of the efferent vesicle, which is chiefly 

 formed from the subcuticular zone; this last also gives rise to the 

 subcuticular musculature. 



New Cestodes from Lamna cornubica.* — Prof. P. J. Van Beneden 

 describes two new cestodes found in the intestine of this Shark. One, 

 which is called DinohotJirium septaria, resembles when contracted in 

 alcohol a shell of the genus Septaria, and appears to have no hooks ; the 

 other is called Diplohothrium simile, on account of the similar suckers 

 which are united in pairs. Both forms have a complete septum between 

 the two pairs of suckers, and this has at its tip four pieces which appear 

 to furnish points of attachment for the muscular layer. The other host 

 of these parasites is not yet known ; they both belong to the group of 

 Phyllobothriidae, all of which are, so far as is yet known, parasitic in 

 plagiostomous fishes. 



Echinodermata . 



Embryology of Echinoderms.t — In his present paper Mr. H. Bury 

 confines himself to the bilateral symmetrical stage, which is more or 

 less clearly represented in all Echinoderm-larvse, and to which Semon 

 has recently given the name of Dipleurula. He deals, therefore, only 

 with the primary divisions of the coolom, starting from a stage in which 

 at least two enterocoel-pouches are already present, with the development 

 and connections of the hydrocoel, and with the skeleton so far as it is 

 developed. 



* Bull. Acad. E. Sci. Belg., lix. (1889) pp. 68-74 (1 pi.), 

 t Quart. Journ. Micr. Sci., xxix. (1889) pp. 409-49 (3 pis.). 



