ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 757 



Development of Fresh-water Dendrocoela.* — M. P. Hallez gives a 

 notice of liis observations on the early stages of the development of fresh- 

 water Dendrocoela. The ova, in the ovary, are not si>herical, but have a 

 long axis parallel to the longitudinal axis of the Planarian, and they are 

 alecithal. Impregnation is effected in the uterus ; when the eggs descend into 

 the genital cloaca they become surrounded by about twenty vitelline cells, 

 arranged radially. M. Hallez thinks that his observations are sufficiently 

 numerous to justify him in denying the presence of polar globules. The 

 following are the changes undergone by the nucleus ; the filaments of 

 chromatin are at the periphery, they become arranged on an equatorial 

 plane, they separate and form a wedge whose axis runs along that of the 

 egg, the wedge being formed of eight meridian filaments of chromatin ; 

 the filaments then become more delicate and separate at the equator, they 

 then become drawn towards the poles, and afterwards become sinuous. 



As far as the 8-stage the blastomeres are equal ; after it the radial 

 vitelline cells begin to shed out a finely granular protoplasmic substance 

 which filters between each of the blastomeres and forms a special medium 

 for it. In the later stages the segmentation cavity becomes filled by this 

 fluid, which goes on increasing as the blastomeres multiply. When the 

 cells which have primitively surrounded the egg have disappeared, those 

 which are nearest the embryo come to their aid, and take their place ; the 

 homogeneous mass can only be considered as a medium, for it takes no part 

 in the formation of the embryo. 



Helminthological Observations.f — Dr. F. Zschokke examined, during 

 his stay at Naples, 72 species of fishes, of which 53 were infested with 

 parasites. Of 257 fish only 73 were quite free. Parasites are more common 

 in Selachians than in Teleosteans. 77 species of parasites were found : 

 38 Cestodes, 16 Trematodes, 3 Acanthocephali, and 20 Nematodes. The 

 first were found almost exclusively in Sharks and Eays ; of 4 species the 

 larval stage was found in Teleosteans, and the adults in Cartilaginous fishes. 

 The Nematodes were more common in bony fishes. The author cannot 

 agree with Di*. Oerley's generalization that a striking peculiarity of Selachian 

 Cestodes is their small size, or that the length of the parasite is in inverse 

 relation to the size of its host. 



S. Incertse Sedis. 



Ectoparasitic Rotifers from the Bay of Naples.^ — Dr. L. Plate gives 

 an account of the Seisonidse, of which as yet only two genera, Seison and 

 SaccohdeUa, are known. A new (third) genus is now formed, which it is 

 proposed to call Paraseison. There is no hindgut in either sex. The wheel- 

 organ may be reduced to a few tactile setae. There are two flask-shaped 

 glands in the hinder part of the head. The gonads are placed laterally or 

 dorsally to the stomach. The ductus ejaculatorius of the male has smooth 

 walls, and there are numerous flask-shaped spermatophores. There is no 

 Bucking disc to the tail, but the hinder pule of the body has the form of a 

 hemisphere, which is beset with a row of small denticles, among which the 

 attaching glands open. Four species of this new genus are described, under 

 the names of P. asplanchnus, nudus, proboscidetis, and ciliatus. 



As will be seen from the above account, Paraseison difiers from Seison 

 in a number of details, but is at the same time clearly a close ally. The 

 relations of the two to SaccohdeUa cannot, with our present slight knowledge 

 of the third genus, be exactly defined. 



* Comptcs Kendiis, civ. (1887) pp. 1732-5. 



t MT. Zool. Stat. Neapel, vii. (1887) pp. 264-71. J Ibid., pp. 234-63 (1 pi.). 



