ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 245 



pairs to one of the principal ganglia of the scolex ; the commissure 

 which connects these last together presents at its centre a considerable 

 enlargement which the author calls the central ganglion ; the com- 

 missure may be called the principal commissure. From the central 

 ganglia, and perpendicularly to the commissure, there are given off 

 two other commissures, which bifurcate, and end in a pair of secondary 

 ganglia ; these are connected by four descending branches. 



The commissure, which is bifurcated at either end, may be known 

 as the transverse commissure. Each of the principal ganglia gives 

 off posteriorly three nerves, of which the median is stronger than the 

 lateral; the six nerves thus formed extend through the whole chain 

 of proglottids ; each lateral ganglion gives off four other filaments, 

 two of which pass to the suckers. 



All the nerves which unite the principal ganglia to the ganglionic 

 swellings of the longitudinal nerves form together an octagonal figure 

 to which the name of superior polygonal commissure is ajDplied; 

 below this is another or inferior polygonal commissure. 



From the histological point of view it is to be noted that the 

 nerve-fibres are not separated by any proper envelope from the 

 parenchyma which they traverse ; true ganglionic cells were found 

 only in the scolex, and the other nervous cells have small oval 

 nuclei. 



From the point of view of comparative anatomy the system now 

 described is difficult to explain ; the polygonal commissures cannot, 

 in consequence of the presence of the central ganglion, be regarded as 

 an oesophageal ring. 



Polycladidea.* — The second half of A. Lang's magnificent work 

 on these worms has appeared ; the first half, it will be remembered,'|" 

 stopped in the course of an account of the male generative system. 

 In the Planoceriilse these organs present some remarkable variations 

 in structure. The female apparatus consists of (1) numerous 

 ovaries which lie scattered through the lateral areje, of (2) oviducts, 

 (3) uterus, and (4) female copulatory apparatus. In many cases 

 special glands are connected with the uterus, which may be distin- 

 guished as the accessory uterine glands. There are also various 

 kinds of fixing organs which are of use either in copulation, or in the 

 deposition of the ova. With this the first portion of the treatise 

 ends. 



The second deals with the ontogeny of the Ehabdoccslida, where, 

 among others, the following results are stated : — 



The first four blastomeres are not equal, and a bilateral symmetry 

 is already indicated ; the largest blastomere corresponds to the 

 hinder end, the next largest to the front end, and the two smallest to 

 the right and left sides of the embryo. The subsequent cleavage of 

 these blastomeres and of their descendants takes place spirally around 

 the primary axis of the egg ; it is quite rhythmical in the sense that 

 the descendants divide in the order of their size. The first four 



* Fauna u. Flora des Golfes von Neapel, si. (188i) pp. 241-6S6 (15 pis.), 

 t See this Journal, iv. (1884) pp. 385. 



