992 SUMMARY OF CURRENT RESEARCHES RELATING TO 



agrei ment with what wo know of its development. When the supply 

 of food was abundant a now aet of fissiparous reproduction is to be 

 observed, before the daughter-bud lias attained the proportions of tho 

 parent. On the other hand, if the amount of food is reduced or 

 altogether withdrawn, reproduction by division completely ceases. 

 In conclusion, the author reminds us that Dr. v. Kennel observed 

 transverse division in an undetermined sj ecies of l'lanarian, which 

 was found in Trinidad. 



Structure and Metamorphosis of Pilidium.* — rrof. W. Salensky 

 commences with an account of the histology of this Nemcrtine larva ; 

 he distinguishes an outer convex surface and umbrella from the 

 inner concave subumbrella ; these differ considerably in structure, and 

 are separated by a circlet of cilia. The space between them is filled 

 by a gelatinous mass which contains mesenchym-cells. 



The preoral portion of the larva is distinguished by the delicacy 

 of its epidermal layer, the cells of which are flat and quite trans- 

 parent ; their contents are clear cell-substance and finely granular 

 protoplasm, the epidermis of the oral part of the larva (subumbrella) 

 is very much thicker. The author agrees with Butschli in denying 

 that the frontal pit is the central organ of the nervous system, 

 although it is the homologue of the similarly placed pit in other 

 vermian larvre ; but it is distinguished from the fully developed 

 frontal plate by its simpler structiu^e, so that it may be looked upon 

 as a kind of rudimentary frontal plate. It consists in Pilidium of a 

 layer of cylindrical or spindle-shaped cells, each of which is divided, 

 by the character of its protoplasm, into an upper and a lower half. 

 The base of the pit is a fine structureless membrane, whence bundles 

 of fibres take their origin ; some of these fibres appear to be muscular, 

 and some to be nervous. 



The ciliated circlet is more complicated in structure than is 

 generally supposed ; it consists of marginal and ciliated cells, and 

 of a provisional but very well developed nervous system ; this last 

 appears to be completely homologous with the nerve-ring discovered 

 by Kleinenberg in Annelids, but is distinguished from it by the 

 greater histological differentiation of its elements. No plexus could 

 be made out in the subumbrella. Correlated with its higher differ- 

 entiation is the more complicated anatomical structure of the ring ; 

 as it passes into the later?! lobes of the trunk portion, there are gan- 

 glionic swellings, which forcn a kind of central apparatus for the larva. 



The mesodermal structures are the mesenchym cells and the 

 muscular fibres ; the latter may be arranged in three groups, — the pair 

 of large retractors of the frontal pit, the muscular layer of the sub- 

 umbrella, and the two large muscles of the lateral lobes. The 

 enteric canal consists of an oesophagus and gut, and both are differ- 

 entiated rather early ; the constituent cells of the former are clearer 

 than those of the latter, and less complicated in structure. 



The earliest rudiments of the nervous system appear in the form 

 of two ectodermal thickenings, which are developed on either side of 



* Zeitschr. f. Wiss. Zool., xliii. (1886) pp. 481-511 (2 pis.). 



