ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 887 



external pair of pharyngeals ; they traverse a pair of small pharyngeal 

 ganglia, as observed by Sars in Porcellio. 



He also considers the structure of the central organs and the 

 sense-organs. The medullary substance of the former, termed by him 

 granular-reticulate substance, and described by Leydig, Dietl, and 

 Krieger as a nervous reticulum, is considered to be made up of a 

 nervous reticulum combined with connective tissue. The reti- 

 culation consists of fibrils derived from the peripheral nerves and 

 from the central nerve-cells. The large nerve-cells have two processes 

 which come from the same pole ; one forms a nerve-fibre directly, the 

 other merges in the reticulum. The small nerve-cells have each a 

 single process, which breaks up in the reticulum. These ganglion- 

 cells, which occur at the point of issue of the nerves, are bipolar. 

 After describing minutely the structure of the brain, the author 

 proceeds to determine the analogy which the cerebral lobes bear to 

 those of insects. In speaking of the sense-organs, he alludes to the 

 " rhabdum " (bacillar layer) and the " retinula." The compound eye 

 consists of the dioptric part, composed of a biconvex corneal lens 

 and a crystalline cone, and the nervous part, excluding the fasciculate 

 optic nerve, which is usually full of nerve-cells. In the nervous parts 

 may be distinguished, according to Grenacher, the retinula and 

 " rhabdum " ; the former is made up of five long cells, coloured black 

 by pigment, alternating with the radii of the rhabdum, and proceeding 

 from the fibres of the optic nerve. He admits, with Grenacher, that 

 the rhabdum is a cuticular production of the cells of the retina. The 

 problem of the homology between the retina of the Arthropods and 

 that of Vertebrata is as yet insoluble, for the relations of the rods and 

 cones to the other elements and to the nerve-fibres in the former group 

 are not as yet understood. 



Vermes. 



Organization of Terrestrial Lumbricina.* — Prof. E. Perrier here 

 deals with the genus Pontoclrilus, two species of which, P. littoralis 

 Grube, and P. Marionis n. sp., are found in France ; the latter in- 

 habiting debris wetted by salt water, has for three years lived in 

 mud, &c., watered with fresh water. The forms of this genus are 

 distinguished from Lumhrici by the postclitellar position of their 

 generative orifices. 



One of the most important chapters is that which deals with the 

 segmental organs ; they are not found in their normal condition until 

 the fifteenth segment is reached, so that they are absent from the seg- 

 ments in which are contained the testicles, ovaries, oviducts, or anterior 

 half of the efferent canals. As, however, they coexist in four rings 

 with these last, and where the " prostatic gland " is developed, it is 

 obvious that this gland is not a modified segmental organ. The first 

 four segmental organs differ in structure from the rest ; they are 

 formed by a coiled glandular tube, which is ciliated internally, and 

 opens by a narrower piece into the preceding segment. From the 

 nineteenth segment backwards the glandular and vibratile portions are 



'' Arch. Zool. Expcr. ct Gen., ix, (1881) pp. 175-249 (6 pis.). 



