ZOOLOGY AND BOTAKY, MICROSCOPY, ETC. 197 



and posterior longitudinal nerves, and which are connected by motor 

 transverse commissures. The sensory swellings are more dorsal and 

 anterior in position ; they give off the sensory nerves, and are likewise 

 connected by commissures. Between these two sets there is a sensori- 

 motor commissure. Histological structure no less than anatomical 

 arrangement reveals the higher grade of development seen in the 

 marine as compared with the other forms ; the sensory are distin- 

 guished from the motor nerves by being invested in a continuous layer 

 of ganglionic cells. 



Structure of Gunda segmentata, and the Relationships of the 

 Platyhelminthes with^the Ccelenterata and Hirudinea.* — Dr. Arnold 

 Lang commences by revising the classification of the lower Platyhel- 

 minthes : he would drop the term Turbellaria, and adopt in its place 

 three orders, each of them the equivalent of the Trematoda, Cestoda, 

 or Nemertinea ; the dendrocoelous Turbellaria are either monogono- 

 porous, or digonoporous, and for them he proposes the terms of 

 Polyclades and Triclades, while the third order would be called the 

 Ehabdocoela. 



Gunda segmentata is a delicate marine Planarian about 6 mm. long, 

 and very active ; after giving a technical description of the species, the 

 author passes to its epithelial layer, some of the cells of which are 

 filled with the small characteristic rods, while others, on the ventral 

 side, form a zone, which is broadest at the anterior end of the body ; 

 these attaching cells project considerably beyond the rest, and their 

 free surface is roughened. The enteric system receives the name of 

 the coelenteric apparatus, inasmuch as the author is convinced that it 

 is the homologue of the ccslenteric apparatus of the Ccelenterata, and 

 of the enteron and coelom of the Enterocoela. In all essential points it 

 agrees with that of the other Triclades ; the mouth leads into the 

 so-called proboscis cavity, from the walls of which are developed 

 muscular folds which project into the cavity, and form the proboscis, 

 in the fashion of a diaphragm. The cavity communicates by an orifice 

 with another cavity, which is not, as is the former, lined by ectoderm, 

 but by endoderm ; from this there are given off the branches of the 

 intestine, the anterior of which lies in the middle line, and ends 

 blindly at the anterior end of the body. The two lateral primary 

 branches lie close to the sides of the proboscis-sheath, and end 

 blindly at the hinder end of the body; from these three primary 

 branches are given off secondary diverticula, the coelomic diverticula 

 of the enteron ; these agree in all essential points with the paired 

 cavities of the enterocoele of higher forms ; they are generally 

 unbranched, or are forked at their peripheral ends. There is no 

 special musculature for the walls of the intestine ; in the enteric cells 

 we may sometimes see large vacuoles during life ; these are called 

 the excretory vacuoles. If we compare the above account with the 

 arrangements which obtain in the Cieno'pliora, we find there that 

 the so-called stomach is lined with ectoderm, and is provided with 

 glandular ridges, that the succeeding cavity is lined by endoderm, 



* MT. Zool. Stat. Neapel, iii. (1881) pp. 187-252 (3 pis.). 



