386 SUMMARY OF CURRENT RESEARCHES RELATING TO 



leptidsa, and Prosthiostomidas. He arranges the bibliograpliical por- 

 tion under epochs ; the first of these begins with O. F. Miiller, 1774 

 (or Stroem, 1768), and ends with Mertens 1832. Mertens begins and 

 Quatrefages (1845) ends the second period ; the third ends with 

 Keferstein in 1868 ; and the fourth with Graff in 1882. In all, we 

 have the titles of 153 books and papers. 



The work proper commences with an account of the methods of 

 investigation; the best preparations were obtained by the following 

 means : — 



Prepared animals were placed for from 8 to 14 days in picro- 

 carmine ; much of the picrin was then removed by alcohol of 70 per 

 cent. ; they were then placed for from one to four days in Grenacher's 

 borax-carmine, and then in feebly acidulated alcohol. In this way 

 the protoplasm was distinctly coloured by picrocarmine, and the nuclei 

 by the borax-carmine, while the long-continued action of the former 

 produced a slight maceration, which was an extraordinary assistance 

 in making out the boundaries of the cells. Mayer's cochineal method 

 is the best for the investigation of glands. As usual, Beale's carmine 

 was very successfully used. 



A general review of the organization of the Polycladidea leads to 

 a series of chapters in which the different organs are discussed ; the 

 epithelial investment, which is always very distinctly marked, and 

 consists of more or less high cylindrical cells, is separated from the 

 tissues of the body by a resisting basal membrane, and is always 

 covered with closely packed and relatively short cilia, which are set 

 on a resisting cortical layer of the cells, which may be regarded as 

 the cuticle. 



Contrary to what happens in most Triclads and all Rhabdocoelids, 

 the " rhabdites " always lie in the epithelium and never in the paren- 

 chyma of the body ; and this is the most primitive arrangement. The 

 mucous rods or pseudorhabdites are next considered ; those found in 

 the Triclads differ in many important points from the similarly named 

 parts found by Graff in the Ehabdocoelidfe. Some forms are very 

 elegantly coloured. True nematocysts are known in one species only, 

 and the calcareous bodies described by Schmarda do not seem to have 

 a real existence. 



In the next chapter the dermomuscular system is first dealt 

 with ; the external layer of diagonal fibres on one side of the body is 

 shown to be the direct continuation of the internal layer of the 

 opposite side. Suckers are of two kinds ; the first, found in all the 

 Cotylea, is to be distinguished from those which, in the Leptoplanidte, 

 lie between the generative orifices, and have clearly a relation to the 

 reproductive function. Dorsoventral muscular fibres are ordinarily 

 well developed. All the fibres of the Polycladidea are thin, elongated, 

 more or less highly refractive, and exhibit no differentiation into an 

 axial substance and a peripheral layer. The dorsoventral fibres are 

 delicately branched at either end, but this would not seem to be the 

 case with those of the dermal system, except in the walls of the 

 suckers. 



The body parenchyma is nest described, and it is stated that 



